10l: the ${prefix} variable should not be expanded when setting path defaults.

Originally committed as revision 5742 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -327,10 +327,10 @@ TMPH="${TMPDIR1}/ffmpeg-conf-${RANDOM}-$$-${RANDOM}.h"
 logging="yes"
 logfile="config.err"
 prefix="/usr/local"
-libdir="${prefix}/lib"
-incdir="${prefix}/include/ffmpeg"
-mandir="${prefix}/man"
-bindir="${prefix}/bin"
+libdir='${prefix}/lib'
+incdir='${prefix}/include/ffmpeg'
+mandir='${prefix}/man'
+bindir='${prefix}/bin'
 cross_prefix=""
 cross_compile="no"
 cc="gcc"