[C++] Define __STDC_CONSTANT_MACROS / __STDC_LIMIT_MACROS for stdint.h

With some toolchains, building in C++ mode stumbles on many instances
of:

 In file included from ../../src/gdb/../include/splay-tree.h:43:0,
                  from ../../src/gdb/dcache.c:26:
 build-gnulib/import/inttypes.h:61:3: error: #error "This file assumes that 'int' has exactly 32 bits. Please report your platform and compiler to <bug-gnulib@gnu.org>."
  # error "This file assumes that 'int' has exactly 32 bits. Please report your platform and compiler to <bug-gnulib@gnu.org>."
    ^
 make: *** [dcache.o] Error 1

That's:

 #if !(INT_MIN == INT32_MIN && INT_MAX == INT32_MAX)
 # error "This file assumes that 'int' has exactly 32 bits. Please report your platform and compiler to <bug-gnulib@gnu.org>."
 #endif

I see it when cross building for --host=x86_64-w64-mingw32 using
Fedora 20's g++ (gcc version 4.8.4 20141219 (Fedora MinGW
4.8.4-1.fc20)), Simon reports seeing this on several cross compilers
too.

The issue is that on some hosts that predate C++11, when using C++ one
must define __STDC_CONSTANT_MACROS/__STDC_LIMIT_MACROS to make visible
the definitions of INTMAX_C / INTMAX_MAX etc.

This was a C99 requirement that later C++11 -- the first to define
stdint.h -- removed, and then C11 removed it as well.

https://www.gnu.org/software/gnulib/manual/html_node/stdint_002eh.html
says that gnulib's stdint.h fixes this, but because we run gnulib's
configure tests with a C compiler, gnulib determines that mingw's
stdint.h is C99-compliant, and doesn't actually replace it.  Actually,
even though configuring gnulib with a C++ compiler does result in
gnulib replacing stdint.h, the resulting replacement is broken for
mingw, because it defines uintptr_t incorrectly.  I sent a gnulib
patch upstream to fix that, here:

  https://lists.gnu.org/archive/html/bug-gnulib/2015-11/msg00004.html

but then even with that, gnulib still stumbles on other
configured-with-C++-compiler problems.

So for now, until gnulib + C++ is fixed upstream and then gdb's copy
is updated, which may take a while, I think it's best to keep
configuring gnulib in C, and define
__STDC_LIMIT_MACROS/__STDC_CONSTANT_MACROS ourselves, just like C99
intended.

gdb/ChangeLog:
2015-11-17  Pedro Alves  <palves@redhat.com>

	* common/common-defs.h (__STDC_CONSTANT_MACROS)
	(__STDC_LIMIT_MACROS): Define before including stdint.h.

2 files changed, 21 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index fbfe4ad..d326b9c 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,5 +1,10 @@
 2015-11-17  Pedro Alves  <palves@redhat.com>
 
+	* common/common-defs.h (__STDC_CONSTANT_MACROS)
+	(__STDC_LIMIT_MACROS): Define before including stdint.h.
+
+2015-11-17  Pedro Alves  <palves@redhat.com>
+
 	* windows-nat.c (handle_exception): Return 0 for first chance
 	exceptions.
 	(get_windows_debug_event): Adjust.
diff --git a/gdb/common/common-defs.h b/gdb/common/common-defs.h
index 2be0d7d..548fe42 100644
--- a/gdb/common/common-defs.h
+++ b/gdb/common/common-defs.h
@@ -31,7 +31,23 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <stddef.h>
+
+/* From:
+    https://www.gnu.org/software/gnulib/manual/html_node/stdint_002eh.html
+
+   "On some hosts that predate C++11, when using C++ one must define
+   __STDC_CONSTANT_MACROS to make visible the definitions of constant
+   macros such as INTMAX_C, and one must define __STDC_LIMIT_MACROS to
+   make visible the definitions of limit macros such as INTMAX_MAX."
+
+   gnulib doesn't fix this for us correctly yet.  See:
+     https://lists.gnu.org/archive/html/bug-gnulib/2015-11/msg00004.html
+
+   Meanwhile, explicitly define these ourselves, as C99 intended.  */
+#define __STDC_CONSTANT_MACROS 1
+#define __STDC_LIMIT_MACROS 1
 #include <stdint.h>
+
 #include <string.h>
 #include <errno.h>
 #include <alloca.h>