Try building without sasl2. Results in:

-- The following OPTIONAL packages have not been found:

 * Sasl2 , The Cyrus-sasl library. , <http://www.cyrussasl.org/>

-- Configuring done

and an cmake error:
-- Configuring done
CMake Error in src/CMakeLists.txt:
  Found relative path while evaluating include directories of "KF5Ldap":

    "Sasl2_INCLUDE_DIRS-NOTFOUND"

Reproducible: Always

Steps to Reproduce:
1. don't have sasl2 installed
2. try running kdesrc-build

Actual Results:  
fails with error, although all required packages are found

Expected Results:  
fails with "required package missing" or proper optional package

Full cmake log:

# kdesrc-build running: 'cmake' '/home/martin/src/kf5/kde/pim/kldap' '-DCMAKE_CXX_FLAGS:STRING=-pipe -DQT_STRICT_ITERATORS -DQURL_NO_CAST_FROM_STRING -DQT_NO_HTTP -DQT_NO_FTP -Wformat -Werror=format-security -Werror=return-type -Wno-variadic-m
acros -Wlogical-op ' '-DCMAKE_INSTALL_PREFIX=/opt/kf5/'
# from directory: /opt/build/kf5/kde/pim/kldap
-- The C compiler identification is GNU 5.3.1
-- The CXX compiler identification is GNU 5.3.1
-- Check for working C compiler: /usr/lib/icecc/bin/cc
-- Check for working C compiler: /usr/lib/icecc/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/lib/icecc/bin/c++
-- Check for working CXX compiler: /usr/lib/icecc/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
-- Looking for __GLIBC__
-- Looking for __GLIBC__ - found
-- Performing Test _OFFT_IS_64BIT
-- Performing Test _OFFT_IS_64BIT - Success
-- Found Gettext: /usr/bin/msgmerge (found version "0.19.7") 
-- Found PythonInterp: /usr/bin/python (found version "2.7.11") 
-- Found Ldap: /usr/lib/x86_64-linux-gnu/libldap.so;/usr/lib/x86_64-linux-gnu/liblber.so (found version "2.4.42") 
-- Checking for module 'libsasl2'
--   No package 'libsasl2' found
-- Could NOT find Sasl2 (missing:  Sasl2_LIBRARIES Sasl2_INCLUDE_DIRS) 
-- Looking for include file sys/time.h
-- Looking for include file sys/time.h - found
-- Looking for ldap_start_tls_s
-- Looking for ldap_start_tls_s - found
-- Looking for ldap_initialize
-- Looking for ldap_initialize - found
-- Looking for ber_memfree
-- Looking for ber_memfree - found
-- Looking for ldap_unbind_ext
-- Looking for ldap_unbind_ext - found
-- Looking for ldap_extended_operation
-- Looking for ldap_extended_operation - found
-- Looking for ldap_extended_operation_s
-- Looking for ldap_extended_operation_s - found
-- Looking for ldap_extended_operation
-- Looking for ldap_extended_operation - found
-- Looking for ldap_extended_operation_s
-- Looking for ldap_extended_operation_s - found
-- Performing Test COMPILER_HAS_HIDDEN_VISIBILITY
-- Performing Test COMPILER_HAS_HIDDEN_VISIBILITY - Success
-- Performing Test COMPILER_HAS_HIDDEN_INLINE_VISIBILITY
-- Performing Test COMPILER_HAS_HIDDEN_INLINE_VISIBILITY - Success
-- Performing Test COMPILER_HAS_DEPRECATED_ATTR
-- Performing Test COMPILER_HAS_DEPRECATED_ATTR - Success
-- 
-- The following RECOMMENDED packages have been found:

 * Ldap , LDAP (Lightweight Directory Access Protocol) libraries. , <http://www.openldap.org/>
   Needed to provide LDAP functionality in KDE

-- The following REQUIRED packages have been found:

 * ECM (required version >= 5.19.0)
 * Qt5Gui (required version >= 5.6.1)
 * KF5KIO (required version >= 5.19.0)
 * KF5DocTools (required version >= 5.19.0)
 * KF5Mbox (required version >= 5.2.40)
 * KF5Completion (required version >= 5.19.0)
 * KF5WidgetsAddons (required version >= 5.19.0)
 * Gettext
 * PythonInterp
 * KF5I18n (required version >= 5.19.0)
 * Qt5Test

-- The following OPTIONAL packages have not been found:

 * Sasl2 , The Cyrus-sasl library. , <http://www.cyrussasl.org/>

-- Configuring done
CMake Error in src/CMakeLists.txt:
  Found relative path while evaluating include directories of "KF5Ldap":

    "Sasl2_INCLUDE_DIRS-NOTFOUND"



CMake Error in kioslave/src/ldap/CMakeLists.txt:
  Target "KF5::Ldap" contains relative path in its
  INTERFACE_INCLUDE_DIRECTORIES:

    "Sasl2_INCLUDE_DIRS-NOTFOUND"


CMake Error in autotests/CMakeLists.txt:
  Target "KF5::Ldap" contains relative path in its
  INTERFACE_INCLUDE_DIRECTORIES:

    "Sasl2_INCLUDE_DIRS-NOTFOUND"


CMake Error in src/CMakeLists.txt:
  Found relative path while evaluating include directories of "KF5Ldap":

    "Sasl2_INCLUDE_DIRS-NOTFOUND"



CMake Error in src/CMakeLists.txt:
  Found relative path while evaluating include directories of "KF5Ldap":

    "Sasl2_INCLUDE_DIRS-NOTFOUND"



CMake Error in kioslave/src/ldap/CMakeLists.txt:
  Target "KF5::Ldap" contains relative path in its
  INTERFACE_INCLUDE_DIRECTORIES:

    "Sasl2_INCLUDE_DIRS-NOTFOUND"


CMake Error in kioslave/src/ldap/CMakeLists.txt:
  Target "KF5::Ldap" contains relative path in its
  INTERFACE_INCLUDE_DIRECTORIES:

    "Sasl2_INCLUDE_DIRS-NOTFOUND"
    "Sasl2_INCLUDE_DIRS-NOTFOUND"



CMake Error in src/CMakeLists.txt:
  Found relative path while evaluating include directories of "KF5Ldap":

    "Sasl2_INCLUDE_DIRS-NOTFOUND"



CMake Error in kioslave/src/ldap/CMakeLists.txt:
  Target "KF5::Ldap" contains relative path in its
  INTERFACE_INCLUDE_DIRECTORIES:

    "Sasl2_INCLUDE_DIRS-NOTFOUND"


CMake Error in kioslave/src/ldap/CMakeLists.txt:
  Target "KF5::Ldap" contains relative path in its
  INTERFACE_INCLUDE_DIRECTORIES:

    "Sasl2_INCLUDE_DIRS-NOTFOUND"


CMake Error in autotests/CMakeLists.txt:
  Target "KF5::Ldap" contains relative path in its
  INTERFACE_INCLUDE_DIRECTORIES:

    "Sasl2_INCLUDE_DIRS-NOTFOUND"


CMake Error in autotests/CMakeLists.txt:
  Target "KF5::Ldap" contains relative path in its
  INTERFACE_INCLUDE_DIRECTORIES:

    "Sasl2_INCLUDE_DIRS-NOTFOUND"


-- Generating done
-- Build files have been written to: /opt/build/kf5/kde/pim/kldap