[FFmpeg-user] Failed to build build ffmpeg for iOS due to GNU assembler not found

[见良]

Hello every one:
    I want to build ffmpeg(2.5 branch from git) iOS in OSX10.10 using xcode 6.1.1,  I have downloaded gas-preprocessor.pl and copy it to /usr/bin and add execution permission ot it, I configure ffmpeg as follows:
    ./configure --prefix=ffmpeg-sdk-ios-armv7 --target-os=darwin --arch=arm --enable-cross-compile --disable-all --enable-protocol=file --enable-avcodec --enable-avformat --enable-avutil --enable-swresample --enable-swscale --enable-decoder=h264 --enable-decoder=mpeg4 --enable-decoder=aac --enable-encoder=mpeg4 --enable-encoder=aac --enable-demuxer=mov --enable-muxer=mp4 --enable-muxer=mov --enable-asm --enable-pthreads --disable-debug --sysroot="/Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS8.1.sdk" --cc="/Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang" --extra-cflags="-arch armv7 -mfpu=neon -miphoneos-version-min=6.1" --extra-ldflags="-arch armv7 -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS8.1.sdk -miphoneos-version-min=6.1"
    But failed with message:
    GNU assembler not found, install/update gas-preprocessor

    What is wrong with my building process? Please help, thanks!