Add missing int tpye, and avoid calling the undeclared exit function.
Implicit ints and function declarations are language features removed
in C99 and are likely to become unsupported (by default) in future
compilers.

--- a/autoconf/configure.in
+++ b/autoconf/configure.in
@@ -615,12 +615,12 @@
 [#define X_LOCALE 1
 #include <X11/Xlocale.h>
 #include <X11/Xlib.h>
-main() {
+int main() {
 char *p;
 if ((p = XSetLocaleModifiers("@im=none")) != NULL && *p)
-exit (XSupportsLocale() ? 0 : 1);
+return XSupportsLocale() ? 0 : 1;
 else
-exit (1);}
+return 1;}
 ],rxvt_cv_func_xlocale=yes, rxvt_cv_func_xlocale=no,
 AC_MSG_WARN([Define NO_XLOCALE in config.h manually]))])