test if CXXFLAGS/CFLAGS contains the -g or the -O options before adding our -g and...

test if CXXFLAGS/CFLAGS contains the -g or the -O options before adding our -g  and -O2 flags: this avoids duplicates in the GCC command line which just generate confusion

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@58985 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775