Makefile-libostree.am: Work around clang issues with g-ir-scanner
Makefile-libostree.am: Work around clang issues with g-ir-scanner

Work around an issue with clang-7 in Fedora 29 where the
`-fstack-clash-protection` flag is not recognized. Force
the compiler used by `g-ir-scanner` to `gcc`, so that the flag
is recognized.

Related: https://github.com/projectatomic/rpm-ostree/pull/1787#issuecomment-473971585

Closes: #1871
Approved by: jlebon

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree