Android: define required __STDC* macros as cflags

Necessary to fix the following radeonsi building errors:

In file included from external/mesa/src/gallium/drivers/radeonsi/si_blit.c:24:
In file included from external/mesa/src/gallium/drivers/radeonsi/si_pipe.h:29:
In file included from external/mesa/src/gallium/drivers/radeonsi/si_shader.h:71:
In file included from external/llvm/include/llvm-c/Core.h:18:
In file included from external/llvm/include/llvm-c/ErrorHandling.h:17:
In file included from external/llvm/include/llvm-c/Types.h:17:
external/llvm/include/llvm/Support/DataTypes.h:49:3: error: "Must #define __STDC_LIMIT_MACROS before #including Support/DataTypes.h"
  ^
external/llvm/include/llvm/Support/DataTypes.h:53:3: error: "Must #define __STDC_CONSTANT_MACROS before "         "#including Support/DataTypes.h"
  ^
2 errors generated.

[Emil Velikov: add inline comment about the defines]
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/Android.common.mk b/Android.common.mk
index 34db677..98d98ee 100644
--- a/Android.common.mk
+++ b/Android.common.mk
@@ -40,8 +40,13 @@
 	-DPACKAGE_VERSION=\"$(MESA_VERSION)\" \
 	-DPACKAGE_BUGREPORT=\"https://bugs.freedesktop.org/enter_bug.cgi?product=Mesa\"
 
+# XXX: The following __STDC_*_MACROS defines should not be needed.
+# It's likely due to a bug elsewhere, but let's temporarily add them
+# here to fix the radeonsi build.
 LOCAL_CFLAGS += \
 	-DENABLE_SHADER_CACHE \
+	-D__STDC_CONSTANT_MACROS \
+	-D__STDC_LIMIT_MACROS \
 	-DHAVE___BUILTIN_EXPECT \
 	-DHAVE___BUILTIN_FFS \
 	-DHAVE___BUILTIN_FFSLL \