Fix libgltf dbgutil build

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