Linking is enough thanks to `-Werror=int-conversion`.

--- a/configure.ac
+++ b/configure.ac
@@ -146,7 +146,7 @@
 
 	AC_CHECK_FUNC(
 		[strerror_r],
-		[AC_RUN_IFELSE(
+		[AC_LINK_IFELSE(
 			[AC_LANG_SOURCE([[
 				#include <errno.h>
 				#include <string.h>