diff --git a/krb5/include/autoconf.h b/krb5/include/autoconf.h
--- a/krb5/include/autoconf.h
+++ b/krb5/include/autoconf.h
@@ -691,7 +691,18 @@
 #define STDC_HEADERS 1
 
 /* Define to 1 if strerror_r returns char *. */
+/*
+ * glibc defines two versions of strerror_r, the char* returning one is used
+ * when _GNU_SOURCE is defined.
+ */
+#if defined(__linux__) && __has_include(<features.h>)
+#include <features.h>  /* get __GLIBC__ */
+#endif
+#if defined(__GLIBC__) && defined(__USE_GNU)
+#define STRERROR_R_CHAR_P 1
+#else
 /* #undef STRERROR_R_CHAR_P */
+#endif
 
 /* Define if sys_errlist is defined in errno.h */
 #define SYS_ERRLIST_DECLARED 1
diff --git a/tools/build/mk/Makefile.boot b/tools/build/mk/Makefile.boot
--- a/tools/build/mk/Makefile.boot
+++ b/tools/build/mk/Makefile.boot
@@ -38,6 +38,8 @@
 # bsd.sys.mk explicitly turns on -Wsystem-headers, but that's extremely
 # noisy when building on Linux.
 CWARNFLAGS+=	-Wno-system-headers
+# Catch mismatched int/pointer types (e.g. glibc's char* strerror_r prototype)
+CWARNFLAGS+=	-Werror=int-conversion
 CWARNFLAGS.clang+=-Werror=incompatible-pointer-types-discards-qualifiers
 
 # b64_pton and b64_ntop is in libresolv on MacOS and Linux: