title: pkg-config cant find libraries on ffmpeg build (linux)
body: <p>I'm trying to build ffmpeg for android on ubuntu...</p>
  
  <pre><code>-I've cross-compiled all the dependencies I need for my configuration
  -I've set up the configuration parameters
  </code></pre>
  
  <p>but ./configure keep saying </p>
  
  <pre><code>ERROR: librtmp not found using pkg-config
  </code></pre>
  
  <p>I've tested and if I remove <code>--enable-librtmp</code> it will configure complain about other library. The thing is I i build all libs myself. So I'm sure it is present and at right folder. </p>
  
  <p>I'm not linux/compiler specialist and so far i've spend more than 10 days to get everything i need to this project rightly done. I know i can simple modify the configure file under ffmpeg and remove the pkg-config check but this isn't the solution I'm looking for</p>
  
  <p>the command i run is:</p>
  
  <pre><code>./configure --prefix=$PREFIX \
    --arch=${ARCH}\
      --cpu=${CPU} \
      --cross-prefix=$CROSS_PREFIX \
      --enable-cross-compile \
      --sysroot=$SYSROOT \
      --target-os=linux \
      --sysinclude=$NDK/sysroot/usr/include \
      --pkg-config=$(which pkg-config) \
      --pkg-config-flags="--static" \
      --enable-pic \
      --enable-gpl \
      --enable-nonfree \
      \
      --disable-shared \
      --enable-static \
      \
      --enable-ffmpeg \
      --disable-ffplay \
      --disable-ffprobe \
      \
      --enable-librtmp \
      --enable-zlib \
      --enable-mediacodec \
      --enable-libx264 \
      --enable-libfdk-aac\
      --enable-libfreetype\
      --enable-libmp3lame\
      --enable-openssl\
      --enable-libfontconfig\
      --enable-bsf=aac_adtstoasc \
      \
      --disable-doc \
      $ADDITIONAL_CONFIGURE_FLAG
  </code></pre>
  
  <p>I would like to know which tests pkg-config do to detect if any lib is installed? and how can i show it the libs i need are really present</p>
  
  <p>=========UPDATE=========</p>
  
  <p><code>${PREFIX}</code> stands for <code>build_dir</code> under which i've a folder <code>lib/pkgconfig</code> with many .pc files including <code>librtmp.pc</code>. so why it can detect the others but not this one? i checked the content of the file and seams to be right, similar to others at same folder</p>