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

@@ -35,24 +35,30 @@ AC_INIT(configure.ac)
             echo
             ])
 
-    dnl get gcc version
-    AC_MSG_CHECKING([gcc version])
-            gccver=$($CC -dumpversion)
-            gccvermajor=$(echo $gccver | cut -d . -f1)
-            gccverminor=$(echo $gccver | cut -d . -f2)
-            gccvernum=$(expr $gccvermajor "*" 100 + $gccverminor)
-    AC_MSG_RESULT($gccver)
-
-    if test "$gccvernum" -ge "400"; then
-            dnl gcc 4.0 or later
-            CFLAGS="$CFLAGS -Wextra -Werror-implicit-function-declaration"
-    else
-            CFLAGS="$CFLAGS -W"
+    if test `basename $CC` = "clang"; then
+        CFLAGS="$CFLAGS -Wextra -Werror-implicit-function-declaration"
     fi
+    if test `basename $CC` = "gcc"; then
+        dnl get gcc version
+        AC_MSG_CHECKING([gcc version])
+                gccver=$($CC -dumpversion)
+                gccvermajor=$(echo $gccver | cut -d . -f1)
+                gccverminor=$(echo $gccver | cut -d . -f2)
+                gccvernum=$(expr $gccvermajor "*" 100 + $gccverminor)
+        AC_MSG_RESULT($gccver)
+
+        if test "$gccvernum" -ge "400"; then
+                dnl gcc 4.0 or later
+                CFLAGS="$CFLAGS -Wextra -Werror-implicit-function-declaration"
+        else
+                CFLAGS="$CFLAGS -W"
+        fi
 
-    # remove optimization options that break our code
-    # VJ 2010/06/27: no-tree-pre added. It breaks ringbuffers code.
-    CFLAGS="$CFLAGS -Wall -fno-strict-aliasing -fno-tree-pre"
+        # remove optimization options that break our code
+        # VJ 2010/06/27: no-tree-pre added. It breaks ringbuffers code.
+        CFLAGS="$CFLAGS -fno-tree-pre"
+    fi
+    CFLAGS="$CFLAGS -Wall -fno-strict-aliasing"
     CFLAGS="$CFLAGS -Wno-unused-parameter"
     CFLAGS="$CFLAGS -std=gnu99"