QMAKE_LFLAGS_NOUNDEF = <EMPTY> for FreeBSD did not get converted
to CMakeLists.txt. This breaks the build on FreeBSD since environ
is missing from libc, it is available at runtime.
Turn -Wl,-no-undefined into warnings on FreeBSD to fix the build.
Also add a comment based on the one from corelib.pro .
Change-Id: I3835884a2682af0f68a4d65dede3e4e5d4104ac8
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>