Fix previously broken xlc checks, simplifying them in the process.

Originally committed as revision 12474 to svn://svn.ffmpeg.org/ffmpeg/trunk

configure

@@ -1372,7 +1372,7 @@ test -z "$need_memalign" && need_memalign="$mmx"
 
 #Darwin CC versions
 if test $targetos = darwin; then
-    if test -n "`$cc -v 2>&1 | grep -q xlc`"; then
+    if $cc -v 2>&1 | grep -q xlc; then
         add_cflags "-qpdf2 -qlanglvl=extc99 -qmaxmem=-1 -qarch=auto -qtune=auto"
     else
         add_cflags "-pipe"
@@ -1802,7 +1802,7 @@ if enabled small; then
     check_cflags -Os            # not all compilers support -Os
     optimizations="small"
 elif enabled optimizations; then
-    if test -n "`$cc -v 2>&1 | grep -q xlc`"; then
+    if $cc -v 2>&1 | grep -q xlc; then
         add_cflags  "-O5"
         add_ldflags "-O5"
     else