fix -lm check

Originally committed as revision 10620 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1217,8 +1217,6 @@ esac
 
 add_extralibs $osextralibs
 
-check_ldflags -lm
-
 if ! disabled logging ; then
     enabled logging || logfile="$logging"
     echo "# $0 $@" >$logfile
@@ -1547,6 +1545,8 @@ for thread in $THREADS_LIST; do
     fi
 done
 
+check_lib math.h sin -lm
+
 # test for lrintf in math.h
 check_exec <<EOF && enable lrintf || disable lrintf
 #define _ISOC9X_SOURCE  1