Fix build with MinGW tooling. (#514)

Courtesy of @Eli-Zaretskii, these fixes should unblock people from
building tree-sitter with MinGW.

I don't think this is an unreasonable maintenance burden, especially
given the Emacs project's interest in using tree-sitter, but
@maxbrunsfeld gets the final call.

script/build-lib

@@ -10,7 +10,7 @@ fi
 ${CC}                   \
   -c                    \
   -O3                   \
-  -std=c99              \
+  -std=gnu99            \
   $CFLAGS               \
   -I lib/src            \
   -I lib/include        \