Fix mac builds due to unguarded-availability error

Just bump our required base version up, as developing Android already
effectively requires 10.13, and these tools won't be distributed to
end-user developers (like fastboot/adb/aapt/etc are).

toolchain/make/src/file.c:898:9: error: 'clock_gettime' is only available on macOS 10.12 or newer [-Wunguarded-availability]

Test: build
Change-Id: I5aef02e9fc6f91fd04381b8895bf656675ccac51
diff --git a/Android.bp b/Android.bp
index dd77db2..db31f2f 100644
--- a/Android.bp
+++ b/Android.bp
@@ -80,6 +80,17 @@
         },
         darwin: {
             local_include_dirs: ["soong/darwin"],
+
+            // This isn't being distributed to end-users, so bump up our machine
+            // requirements.
+            cflags: [
+                "-mmacosx-version-min=10.13",
+                "-UMACOSX_DEPLOYMENT_TARGET",
+                "-DMACOSX_DEPLOYMENT_TARGET=10.13",
+            ],
+            ldflags: [
+                "-mmacosx-version-min=10.13",
+            ],
         },
         not_windows: {
             srcs: [