The proxy autoconfiguration portion of KIO fails to build against the musl libc due to _BSD_SOURCE and _GNU_SOURCE both being undefined.  This causes the symbol 'setgroups' to be hidden as it is not specified in POSIX.1-2008.  It also causes NI_MAXHOST and IFF_LOOPBACK to be undefined.

I suggest adding a check for setgroups to ConfigureChecks.cmake in the kpac/ directory and defining _BSD_SOURCE if it is not available, but that is just a suggestion.  I am not sure the desired style in the KIO project.

Reproducible: Always

Steps to Reproduce:
1. Use musl libc.
2. Attempt to build KIO's proxy autoconfiguration support.

Actual Results:  
/usr/src/kde-frameworks/kio-5.21.0/work/kio-5.21.0/src/kpac/kpac_dhcp_helper.c: In function ‘set_gid’:
/usr/src/kde-frameworks/kio-5.21.0/work/kio-5.21.0/src/kpac/kpac_dhcp_helper.c:59:9: error: implicit declaration of function ‘setgroups’ [-Werror=implicit-function-declaration]
     if (setgroups(1, &gid) == -1) {
         ^
/usr/src/kde-frameworks/kio-5.21.0/work/kio-5.21.0/src/kpac/kpac_dhcp_helper.c: In function ‘send_request’:
/usr/src/kde-frameworks/kio-5.21.0/work/kio-5.21.0/src/kpac/kpac_dhcp_helper.c:173:19: error: ‘NI_MAXHOST’ undeclared (first use in this function)
     char hostname[NI_MAXHOST];
                   ^
/usr/src/kde-frameworks/kio-5.21.0/work/kio-5.21.0/src/kpac/kpac_dhcp_helper.c:173:19: note: each undeclared identifier is reported only once for each function it appears in
/usr/src/kde-frameworks/kio-5.21.0/work/kio-5.21.0/src/kpac/kpac_dhcp_helper.c:186:30: error: ‘IFF_LOOPBACK’ undeclared (first use in this function)
         if (ifa->ifa_flags & IFF_LOOPBACK) {
                              ^

Expected Results:  
Successful build.