Make configure work with clang

It was mistaking clang for gcc and deciding its version
was too low.

configure.ac

@@ -48,9 +48,23 @@ solaris*)
     ;;
 esac
 
+dnl clang is mostly GCC-compatible, but its version is much lower,
+dnl so we have to check for it.
+AC_MSG_CHECKING([if compiling with clang])
+
+AC_COMPILE_IFELSE(
+[AC_LANG_PROGRAM([], [[
+#ifndef __clang__
+       not clang
+#endif
+]])],
+[CLANG=yes], [CLANG=no])
+
+AC_MSG_RESULT([$CLANG])
+
 dnl If we're using GCC, make sure that it is at least version 3.3.0.  Older
 dnl versions are explictly not supported.
-if test "x$GCC" = xyes; then
+if test "x$GCC" = xyes -a "x$CLANG" = xno; then
     AC_MSG_CHECKING([whether gcc version is sufficient])
     major=0
     minor=0