build: Add -std=gnu99 to libglnx CFLAGS
build: Add -std=gnu99 to libglnx CFLAGS

It uses inline for loop initialisers, which are a C99 feature. Set
-std=gnu99 like ostree.git does for its libglnx submodule.

Based on a patch by Georges Basile Stavracas Neto
<georges.stavracas@gmail.com>.

Signed-off-by: Philip Withnall <29c2a2ac2a854b9cf047275aa9de346d096be2e0@endlessm.com>

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