BaseTools/Source/C: take EXTRA_OPTFLAGS from the caller

BaseTools/Source/C: take EXTRA_OPTFLAGS from the caller

Allow the caller of the top-level makefile either to set EXTRA_OPTFLAGS in
the environment or to pass EXTRA_OPTFLAGS as a macro definition on the
command line. EXTRA_OPTFLAGS extends (and potentially overrides) default C
compilation flags set in the makefiles.

Cc: Liming Gao <liming.gao@intel.com>
Cc: Yonghong Zhu <yonghong.zhu@intel.com>
Ref: https://bugzilla.redhat.com/show_bug.cgi?id=1540244
Contributed-under: TianoCore Contribution Agreement 1.1
Signed-off-by: Laszlo Ersek <lersek@redhat.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>