Entware GCC (8.4.0) installs a supposedly fixed ~limits.h~ (in
~/opt/lib/gcc/x86_64-openwrt-linux-gnu/8.4.0/include-fixed/limits.h~, see
https://gcc.gnu.org/onlinedocs/gcc/Fixed-Headers.html) which is the source of a
number of issues due to missing macro definitions.

Observed issues:
* cpython: missing definition of SSIZE_MAX
* abseil-cpp: missing definition of SSIZE_MAX
* llvm:

--- a/opt/lib/gcc/x86_64-openwrt-linux-gnu/8.4.0/include-fixed/limits.h
+++ b/opt/lib/gcc/x86_64-openwrt-linux-gnu/8.4.0/include-fixed/limits.h
@@ -149,4 +149,11 @@
 # define ULLONG_WIDTH __LONG_LONG_WIDTH__
 #endif

+/*
+  KTKT: Include system/libc limits.h (define _GCC_LIMITS_H_ to avoid recursion
+  in /opt/include/limits.h).
+*/
+#define _GCC_LIMITS_H_
+#include_next <limits.h>
+
 #endif /* _LIMITS_H___ */