build: exit on unsupported host OS for Android

The Android configure script throws an unhelpful error message telling
the user that `toolchain_path` is not defined when the script is run on
an unsupported host OS, exit with a more helpful message instead while
listing the supported host OSes.

android_configure.py

@@ -57,10 +57,12 @@ def patch_android():
 if platform.system() == "Darwin":
     host_os = "darwin"
     toolchain_path = android_ndk_path + "/toolchains/llvm/prebuilt/darwin-x86_64"
-
 elif platform.system() == "Linux":
     host_os = "linux"
     toolchain_path = android_ndk_path + "/toolchains/llvm/prebuilt/linux-x86_64"
+else:
+    print("\033[91mError: \033[0m" + "Unsupported host OS, must be one of: Linux, Darwin")
+    sys.exit(1)
 
 os.environ['PATH'] += os.pathsep + toolchain_path + "/bin"
 os.environ['CC'] = toolchain_path + "/bin/" + TOOLCHAIN_PREFIX + android_sdk_version + "-" +  "clang"