Use pkg-config for libavcodec detection

Use pkg-config to check existence of libavcodec and to get its include path along with cflags and ldflags. Note that there is still a hack present for detecting the 32-bit library location of different Linux systems. For example, Fedora uses lib 32-bit and lib64 for 64-bit.

Also, I could not fully complete the script on my Fedora. I only have 64-bit ffmpeg development libraries. I think there should be a sub-configure for glinject instead of glinject being included in the main (i.e. you don't need libavcodec to build glinject).

Further ideas:
pkg-config can also check the version and if libavcodec comes from ffmpeg or libav. But I think the best option would be an autotools test that checks exactly the functions you need.