configure: address new warning from latest autoreconf

autoreconf from autoconf 2.70 warns that AC_PROG_CC_STDC is obsolete,
recommending to use AC_PROG_CC instead. We already call the latter
macro, and the behavior change appears to have been introduced in
autoconf 2.54 (released 2002-09-13), which is older than our minimum
supported version of 2.66.

Signed-off-by: Patrick McCarty <patrick.mccarty@intel.com>

Author: phmccarty

configure.ac

@@ -2,7 +2,6 @@ AC_PREREQ([2.66])
 AC_INIT([bsdiff], [1.0.4], [patrick.mccarty@intel.com])
 AC_CONFIG_MACRO_DIR([m4])
 AC_PROG_CC
-AC_PROG_CC_STDC
 AC_LANG(C)
 AC_CONFIG_HEADERS([config.h])
 AC_PREFIX_DEFAULT(/usr/local)