Simplify cross-compiling on Mac OS X host machine by copy-paste commands

extensions/ffmpeg/README.md

@@ -31,6 +31,13 @@ FFMPEG_EXT_PATH="${EXOPLAYER_ROOT}/extensions/ffmpeg/src/main"
 NDK_PATH="<path to Android NDK>"
 ```
 
+* Set up host platform ("darwin-x86_64" for Mac OS X):
+
+```
+HOST_PLATFORM="linux-x86_64"
+```
+
+
 * Fetch and build FFmpeg. For example, to fetch and build for armeabi-v7a,
   arm64-v8a and x86 on Linux x86_64:
 
@@ -60,7 +67,7 @@ git clone git://source.ffmpeg.org/ffmpeg ffmpeg && cd ffmpeg && \
     --libdir=android-libs/armeabi-v7a \
     --arch=arm \
     --cpu=armv7-a \
-    --cross-prefix="${NDK_PATH}/toolchains/arm-linux-androideabi-4.9/prebuilt/linux-x86_64/bin/arm-linux-androideabi-" \
+    --cross-prefix="${NDK_PATH}/toolchains/arm-linux-androideabi-4.9/prebuilt/${HOST_PLATFORM}/bin/arm-linux-androideabi-" \
     --sysroot="${NDK_PATH}/platforms/android-9/arch-arm/" \
     --extra-cflags="-march=armv7-a -mfloat-abi=softfp" \
     --extra-ldflags="-Wl,--fix-cortex-a8" \
@@ -72,7 +79,7 @@ make clean && ./configure \
     --libdir=android-libs/arm64-v8a \
     --arch=aarch64 \
     --cpu=armv8-a \
-    --cross-prefix="${NDK_PATH}/toolchains/aarch64-linux-android-4.9/prebuilt/linux-x86_64/bin/aarch64-linux-android-" \
+    --cross-prefix="${NDK_PATH}/toolchains/aarch64-linux-android-4.9/prebuilt/${HOST_PLATFORM}/bin/aarch64-linux-android-" \
     --sysroot="${NDK_PATH}/platforms/android-21/arch-arm64/" \
     --extra-ldexeflags=-pie \
     ${COMMON_OPTIONS} \
@@ -82,7 +89,7 @@ make clean && ./configure \
     --libdir=android-libs/x86 \
     --arch=x86 \
     --cpu=i686 \
-    --cross-prefix="${NDK_PATH}/toolchains/x86-4.9/prebuilt/linux-x86_64/bin/i686-linux-android-" \
+    --cross-prefix="${NDK_PATH}/toolchains/x86-4.9/prebuilt/${HOST_PLATFORM}/bin/i686-linux-android-" \
     --sysroot="${NDK_PATH}/platforms/android-9/arch-x86/" \
     --extra-ldexeflags=-pie \
     --disable-asm \