Show what macros are predefined by your GCC. e.g., for Android, you can rely on -D__ANDROID__

gcc_predefined_macros.sh

#!/bin/sh

echo "Hello, world"

# PC linux
echo | gcc -dM -E - | less

# For the Android NDK toolchains
echo | /opt/android/android-ndk/toolchains/x86-4.4.3/prebuilt/linux-x86/bin/i686-linux-android-gcc -dM -E -

To east the execution of those cross-compilation toolchain commands, you can create a utility script to help you omit the long prefix. e.g., for Android, create an NDK script in your PATH like this:

#!/bin/sh

PREFIX="/opt/android/android-ndk/toolchains/x86-4.4.3/prebuilt/linux-x86/bin/i686-linux-android-"

usage()
{
    echo "USAGE: $0 TOOL ARGS"
}

if [ $# -lt 2 ]; then
    usage
    exit 1
fi

TOOL="$1"
shift

exec "${PREFIX}${TOOL}" "$@"

Remember to chmod +x NDK. Then above command is transformed into:

echo | NDK gcc -dM -E - | less