Use pkg-config to generate the linker flags for libdirac.

We do the same for libschroedinger and already use pkg-config to generate
the CFLAGS for libdirac anyway, so there is no new dependency.
Thanks to Kovensky for reporting breakage on IRC.

Originally committed as revision 19654 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2240,8 +2240,8 @@ check_func truncf
 # these are off by default, so fail if requested and not available
 enabled avisynth   && require2 vfw32 "windows.h vfw.h" AVIFileInit -lvfw32
 enabled libdirac   && add_cflags $(pkg-config --cflags dirac) &&
-                      require  libdirac libdirac_decoder/dirac_parser.h dirac_decoder_init -ldirac_decoder &&
-                      require  libdirac libdirac_encoder/dirac_encoder.h dirac_encoder_init -ldirac_encoder
+                      require  libdirac libdirac_decoder/dirac_parser.h dirac_decoder_init $(pkg-config --libs dirac) &&
+                      require  libdirac libdirac_encoder/dirac_encoder.h dirac_encoder_init $(pkg-config --libs dirac)
 enabled libfaac    && require2 libfaac "stdint.h faac.h" faacEncGetVersion -lfaac
 enabled libfaad    && require2 libfaad faad.h faacDecOpen -lfaad
 enabled libgsm     && require  libgsm gsm.h gsm_create -lgsm