I have some rules in CMake which cause the build dir to be non-empty after a prune.  Running a configure in the build dir, will then be missing the '-G Ninja'.

Reproducible: Always

Steps to Reproduce:
1. Prune
2. Configure


Actual Results:  
Unix Makefile (the default) is selected as the generator:

build> /.../cmake -DCMAKE_INSTALL_PREFIX=/usr/local /home/.../project


Expected Results:  
Have Ninja as the generator:

build> /.../cmake -DCMAKE_INSTALL_PREFIX=/usr/local -G Ninja /home/.../project

In projectbuilders/cmakebuilder/cmakejob.cpp

The command line for cmake is built with or without the '-G' option, if the build dir is detected to be empty (or inexistant):

//if we are creating a new build directory, we'll want to specify the generator
QDir builddir(CMake::currentBuildDir( m_project ).toLocalFile());
   if(!builddir.exists() || builddir.count()==2) {

However, in my case, the build dir is present and non-empty (some wierd reason why, not important for this issue), but make hasn't run yet.

I propose a better check, which is to see if the CMakeCache.txt file is present, which it should *always* be created if cmake hasn't run.  Even if it has, and this file is missing, cmake will run as if it is the first time.  Therefore, the presence of this file is a better check if the build dir is ready in cmake's eyes.