Set correct value for libdir in pkg-config files.

patch by Anssi Hannula, anssi.hannula gmail com

Originally committed as revision 12864 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -2101,7 +2101,7 @@ fi
 
 
 # build pkg-config files
-# FIXME: libdir and includedir are hardcoded and may differ from the real path.
+# FIXME: includedir is hardcoded and may differ from the real path.
 
 pkgconfig_generate(){
 name=$1
@@ -2112,7 +2112,7 @@ requires=$5
 cat <<EOF >$name.pc
 prefix=$prefix
 exec_prefix=\${prefix}
-libdir=\${exec_prefix}/lib
+libdir=$libdir
 includedir=\${prefix}/include
 
 Name: $name