build: Reword C99 compiler check output

Specify the current compiler being used, and shorten the message
checking for an option to enable C99 support.

debian/changelog

@@ -4,6 +4,7 @@ dpkg (1.17.3) UNRELEASED; urgency=low
     on systems with old g++ compilers not accepting -Wc++11-compat.
   * Move DPKG_C_C99 call just after AC_PROG_CC, so that subsequent checks
     can take advantage of the possibly set flags to enable C99 features.
+  * Improve configure C99 compiler check output.
 
  -- Guillem Jover <guillem@debian.org>  Thu, 05 Dec 2013 06:50:31 +0100
 

m4/dpkg-compiler.m4

@@ -153,11 +153,11 @@ AC_DEFUN([DPKG_TRY_C99],
 # ----------
 # Check whether the compiler can do C99
 AC_DEFUN([DPKG_C_C99],
-[AC_CACHE_CHECK([whether compiler supports C99 features], [dpkg_cv_c99],
+[AC_CACHE_CHECK([whether $CC supports C99 features], [dpkg_cv_c99],
 	[DPKG_TRY_C99([dpkg_cv_c99=yes], [dpkg_cv_c99=no])])
 AS_IF([test "x$dpkg_cv_c99" = "xyes"],
 	[AC_DEFINE([HAVE_C99], 1, [Define to 1 if the compiler supports C99.])],
-	[AC_CACHE_CHECK([what argument makes compiler support C99 features],
+	[AC_CACHE_CHECK([for $CC option to accept C99 features],
 		[dpkg_cv_c99_arg],
 		[dpkg_cv_c99_arg=none
 		 dpkg_save_CC="$CC"