gssapi: define HAVE_LIBGSSAPI_KRB5 in configure.ac

Problem: configure.ac is not setting HAVE_LIBGSSAPI_KRB5
in src/platform.hpp when --with-libgssapi_krb5 is specified

Commit 09e868b7
switched the libgssapi_krb5 check from AC_CHECK_LIB
to AC_SEARCH_LIBS, but neglected to add an AC_DEFINE
for HAVE_LIBGSSAPI_KRB5, thus the GSSAPI code is
never compiled.

Solution: Add missing AC_DEFINE of HAVE_LIBGSSAPI_KRB5.

configure.ac

@@ -427,7 +427,8 @@ AC_ARG_WITH([libgssapi_krb5], [AS_HELP_STRING([--with-libgssapi_krb5],
 # conditionally require libgssapi_krb5
 if test "x$require_libgssapi_krb5_ext" != "xno"; then
     AC_CHECK_HEADERS(gssapi/gssapi_generic.h)
-    AC_SEARCH_LIBS([gss_init_sec_context], [gssapi_krb5 gssapi],,
+    AC_SEARCH_LIBS([gss_init_sec_context], [gssapi_krb5 gssapi],
+        AC_DEFINE(HAVE_LIBGSSAPI_KRB5, [1], [Enabled GSSAPI security]),
         AC_MSG_ERROR(libgssapi_krb5 is needed for GSSAPI security))
 fi