giscanner: Disable warnings arising from -D_FORTIFY_SOURCE -O0

Using distutils.ccompiler means that we are forced to use the CFLAGS
from the system’s Python installation, which may contain
-D_FORTIFY_SOURCE. The user’s environment-provided CFLAGS may contain
-O0 (because they are a developer). These two flags cause a warning when
used together. Silence that warning by passing -Wno-cpp to disable
warnings from #warning preprocessor statements in the generated C code.

It doesn’t seem to be possible to selectively undefine _FORTIFY_SOURCE
or to stop using the compiler flags from distutils.sysconfig,
unfortunately.

https://bugzilla.gnome.org/show_bug.cgi?id=757934