The libgltf configure.ac script changes the autotools user variables
CFLAGS and CXXFLAGS, which is not allowed and can overwrite user
settings (which it does for the -D_GLIBCXX_DEBUG flag).
So this moves the special compile settings to seperate variables and
passes them to AM_CXXFLAGS and AM_CPPFLAGS, so the library actually
gets compiled with libstdc++ debug objects.
Change-Id: I00989f5fb629a6aac43ee5a2eb287b0491a3b86d
liborcus was not building for me with -flto in CFLAGS, I would have to
fix ar somehow.
-flto in LDFLAGS is just to fix the build if the external library does use
another library built by us with -flto: does happen for liborcus and python3.
It's not like we would use -flto for external libraries consistently anyway,
the only exception is icu: no idea why we build with -flto there.
Change-Id: Ia54d619674b8999ce5e4b920ba77b1587c9cf48d
