BaseTools/footer.makefile: expand BUILD_CFLAGS last for C files too
    
    BUILD_CPPFLAGS should be expanded before BUILD_CFLAGS. (The rule for C++
    source files already does this, with BUILD_CPPFLAGS and BUILD_CXXFLAGS.)
    
    This patch doesn't change behavior.
    
    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>