GCC installed from FreeBSD ports doesn't support building PIE executables
and fails with:

/usr/local/bin/ld: /usr/lib/crt1.o: relocation R_X86_64_32 against
`_DYNAMIC' can not be used when making a shared object; recompile with
-fPIC
/usr/lib/crt1.o: error adding symbols: Bad value
collect2: error: ld returned 1 exit status

However, the configure check for '-fPIC -DPIC' doesn't catch that. In
order to catch this case, add '-pie' to CFLAGS in m4/virt-compile-pie.m4
so it could detect lack of PIE support on configure time and don't fail
the build.