DEBUG_GCSAFETY broken with g++ < 4.7

[Bruno H. <br...@cl...>]

Hi Sam,

I need to correct your change of

2016-08-28  Sam Steingold  <sd...@gn...>

	avoid the clang -Wgnu-designator warning
	* lispbibl.d (as_object, as_chart): use the new {.one_o=???}
	syntax instead of GNU old-style field designator extension {one_o:???}
	https://gcc.gnu.org/onlinedocs/gcc-6.2.0/gcc/Designated-Inits.html

The reason is that it introduces compilation errors when one attempts to use
DEBUG_GCSAFETY with g++ < 4.7.

DEBUG_GCSAFETY requires a C++ compiler and defines OBJECT_STRUCT.

Look at this test program:

================================= foo.cc =================================
typedef unsigned long uintP;
typedef signed long sintP;
typedef struct { uintP one_o; } gcv_object_t;
typedef uintP oint;
typedef sintP soint;
typedef gcv_object_t object;
#define as_oint(expr)  ((expr).one_o)

// Works with any GCC but only with G++ >= 4.7. With clang no warning.
#define as_object(o)  ((object){.one_o=(o)})

// Works with any GCC and any G++. With clang it gives "warning: use of GNU old-style field designator extension"
//#define as_object(o)  ((object){one_o:(o)})

extern gcv_object_t a;
extern oint b;

oint foo () { return as_oint(a); }

gcv_object_t bar () { return as_object(b); }
==========================================================================

With G++ 4.6.4 and older:
$ g++ -c foo.cc
foo.cc: In Funktion »gcv_object_t bar()«:
foo.cc:21:30: Fehler: expected primary-expression before »)« token
foo.cc:21:30: Fehler: expected »)« before »{« token
foo.cc:21:30: Fehler: expected »;« before »)« token
foo.cc:21:30: Fehler: expected primary-expression before »)« token
foo.cc:21:30: Fehler: expected »;« before »)« token

The syntax   .one_o=(o)
appears to be more standard: it is described in Wikipedia [1] and the GCC
documentation [2].

The syntax   one_o:(o)
is only documented in the GCC doc [2] and is marked as "obsolete since GCC 2.5".
But with G++ 4.6.4 and older it's the only available syntax.

Bruno

[1] https://en.wikipedia.org/wiki/C_syntax#Designated_initializers
[2] https://gcc.gnu.org/onlinedocs/gcc-6.2.0/gcc/Designated-Inits.html