On FreeBSD, this variable is defined as a common symbol in the object file
/usr/lib/crt1.o, which is injected into the final application by the
compiler. This means when linking any library, 'environ' cannot be found
and we can't use -Wl,-no-undefined on FreeBSD.

I don't know why this wasn't caught before. Most likely, we failed to
pass the linker flag until some recent change to the buildsystem.

qprocess_unix.cpp:279: undefined reference to `environ'

Change-Id: I7a9e11d7b64a4cc78e24ffff142e02dbf188bca5
Reviewed-by: Simon Hausmann <simon.hausmann@theqtcompany.com>
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>