Add MIR_CFLAGS to SDL_CFLAGS so these cflags are added to the sdl2.pc file.
diff --git a/configure.in b/configure.in
index 0261a7a..f7537f0 100644
--- a/configure.in
+++ b/configure.in
@@ -1299,6 +1299,7 @@ AC_HELP_STRING([--enable-mir-shared], [dynamically load Mir support [[default=ma
EXTRA_LDFLAGS="$EXTRA_LDFLAGS $MIR_LIBS"
SUMMARY_video="${SUMMARY_video} mir"
fi
+ SDL_CFLAGS="$SDL_CFLAGS $MIR_CFLAGS"
have_video=yes
fi
fi