compat: fall back on __va_copy if available

Since an obvious implementation of va_list is to make it a pointer
into the stack frame, implementing va_copy as "dst = src" will work on
many systems.  Platforms that use something different (e.g., a size-1
array of structs, to be assigned with *(dst) = *(src)) will need some
other compatibility macro, though.

Luckily, as the glibc manual hints, such systems tend to provide the
__va_copy macro (introduced in GCC in March, 1997).  By using that if
it is available, we can cover our bases pretty well.

Discovered by building with CC="gcc -std=c89" on an amd64 machine:

 $ make CC=c89 strbuf.o
 [...]
 strbuf.c: In function 'strbuf_vaddf':
 strbuf.c:211:2: error: incompatible types when assigning to type 'va_list'
  from type 'struct __va_list_tag *'
 make: *** [strbuf.o] Error 1

Explained-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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