@@ -823,12 +823,10 @@
 			AC_MSG_RESULT(no)
 		])
 		;;
 esac
 
-AC_CHECK_FUNCS(strerror_r)
-
 AC_CHECK_LIB(m, fmod, LIBS="$LIBS -lm")
 AC_CHECK_LIB(complex, creal, TESTS_LIBS="$TESTS_LIBS -lcomplex")
 
 AC_CHECK_FUNC(asprintf, [
 	case "$host" in
@@ -1263,10 +1261,30 @@
 	], [
 		AC_MSG_RESULT(yes)
 		CPPFLAGS="-D_GNU_SOURCE $CPPFLAGS"
 	])
 ])
+
+dnl This check needs to happen after the above, as _GNU_SOURCE can change the
+dnl return type.
+AC_CHECK_FUNCS(strerror_r, [
+	AC_MSG_CHECKING(for return type of strerror_r)
+	AC_TRY_COMPILE([
+		#include <stdio.h>
+		#include <string.h>
+	], [
+		switch (strerror_r(0, NULL, 0)) {
+		case 0:;
+		}
+	], [
+		AC_MSG_RESULT(int)
+	], [
+		AC_MSG_RESULT(char *)
+		AC_DEFINE(STRERROR_R_RETURNS_CHARP, 1,
+			[Whether strerror_r returns char *])
+	])
+])
 
 AC_CHECK_HEADERS(sys/utsname.h)
 AC_CHECK_FUNCS(uname)
 
 case "$host_os" in