configure: differentiate gcc and clang options

The version checking was made similarly for clang and gcc. This
patch modifies this to check on compiler name. This way we can
avoid to set march=native which is not supported by clang on
some system.
At the same time, this fix the annoying warning about no-fp-tree
being unsupported by clang.

configure.ac

@@ -35,6 +35,10 @@ AC_INIT(configure.ac)
             echo
             ])
 
+    if test `basename $CC` = "clang"; then
+        CFLAGS="$CFLAGS -Wextra -Werror-implicit-function-declaration"
+    fi
+    if test `basename $CC` = "gcc"; then
         dnl get gcc version
         AC_MSG_CHECKING([gcc version])
                 gccver=$($CC -dumpversion)
@@ -52,7 +56,9 @@ AC_INIT(configure.ac)
 
         # remove optimization options that break our code
         # VJ 2010/06/27: no-tree-pre added. It breaks ringbuffers code.
-    CFLAGS="$CFLAGS -Wall -fno-strict-aliasing -fno-tree-pre"
+        CFLAGS="$CFLAGS -fno-tree-pre"
+    fi
+    CFLAGS="$CFLAGS -Wall -fno-strict-aliasing"
     CFLAGS="$CFLAGS -Wno-unused-parameter"
     CFLAGS="$CFLAGS -std=gnu99"