Configure: fixed clang version detection (closes #1539).

While 325b3042edd6 fixed it on MINIX, it broke it on systems
that output the word "version" on several lines with "cc -v".
The fix is to only consider "clang version" or "LLVM version"
as clang version, but this time only using sed(1).

auto/cc/clang

@@ -6,7 +6,8 @@
 
 
 NGX_CLANG_VER=`$CC -v 2>&1 | grep 'version' 2>&1 \
-                           | sed -e 's/^.* version \(.*\)/\1/'`
+                           | sed -n -e 's/^.*clang version \(.*\)/\1/p' \
+                                    -e 's/^.*LLVM version \(.*\)/\1/p'`
 
 echo " + clang version: $NGX_CLANG_VER"