Fix gdb C++ compilation on Solaris (PR build/20712)

Fix gdb C++ compilation on Solaris (PR build/20712)

gdb 7.12 doesn't compile as C++ (tried with g++ 4.9) on Solaris (tried
10 and 12, sparc and x86).  The following patch (relative to the 7.12
release, though I expect most if not all issues to be present on trunk,
too) fixes this.

Only a few of the changes bear explanation:

* Initially, compilation failed whereever defs.h. was included:

In file included from /vol/src/gnu/gdb/gdb-7.12/gdb/gdb.c:19:0:
/vol/src/gnu/gdb/gdb-7.12/gdb/defs.h:630:33: error: 'double atof(const char*)' conflicts with a previous declaration
 extern double atof (const char *); /* X3.159-1989  4.10.1.1 */
                                 ^
In file included from /usr/include/stdlib.h:17:0,
                 from build-gnulib/import/stdlib.h:36,
                 from /vol/src/gnu/gdb/gdb-7.12/gdb/common/common-defs.h:32,
                 from /vol/src/gnu/gdb/gdb-7.12/gdb/defs.h:28,
                 from /vol/src/gnu/gdb/gdb-7.12/gdb/gdb.c:19:
/vol/gcc-4.9/lib/gcc/i386-pc-solaris2.10/4.9.0/include-fixed/iso/stdlib_iso.h:119:15: note: previous declaration 'double std::atof(const char*)'
 extern double atof(const char *);
               ^

  This is due to this gem in gdb/defs.h which seems to have been present
  like forever:

#ifndef atof
extern double atof (const char *);	/* X3.159-1989  4.10.1.1 */
#endif

  In the Solaris headers, the appropriate functions are in namespace std,
  thus the conflict.  I've wrapped the defs.h declaration in !__cplusplus
  to avoid this; perhaps it can go completely instead.

* All the casts are necessary to appease g++ and should be pretty
  obvious.

* The sol-thread.c changes are here to handle

/vol/src/gnu/gdb/gdb-7.12/gdb/sol-thread.c: In function 'void _initialize_sol_thread()':
/vol/src/gnu/gdb/gdb-7.12/gdb/sol-thread.c:1252:36: error: invalid conversion from 'void*' to 'void (*)(int)' [-fpermissive]
   if (!(p_##X = dlsym (dlhandle, #X))) \
                                    ^
/vol/src/gnu/gdb/gdb-7.12/gdb/sol-thread.c:1255:3: note: in expansion of macro 'resolve'
   resolve (td_log);
   ^

  and are modeled after linux-thread-db.c (try_thread_db_load_1).

The patch allowed both 32 and 64-bit C++ builds on sparc-sun-solaris2.10
and i386-pc-solaris2.10 to complete.  The resulting binary hasn't seen
more than a smoke test (invoke it on itself, b main, run) yet.

When investigating the failure to detect -static-libstdc++
support (more below), I found two more issues which only show up with
-Werror:

/vol/src/gnu/gdb/gdb/local/gdb/procfs.c: In function 'ssd* proc_get_LDT_entry(procinfo*, int)':
/vol/src/gnu/gdb/gdb/local/gdb/procfs.c:2487:19: error: variable 'old_chain' set but not used [-Werror=unused-but-set-variable]
   struct cleanup *old_chain = NULL;
                   ^

Unless I'm mistaken, you need to run do_cleanups on every return from
the function.

Afterwards, I ran a 32-bit compilation, which (after adding
--disable-largefile to avoid

In file included from /usr/include/sys/procfs.h:28:0,
                 from /vol/src/gnu/gdb/gdb/local/gdb/i386-sol2-nat.c:23:
/usr/include/sys/old_procfs.h:39:2: error: #error "Cannot use procfs in the large file compilation environment"
 #error "Cannot use procfs in the large file compilation environment"
  ^

and two more instances) revealed

/vol/src/gnu/gdb/gdb/local/gdb/top.c: In function 'void gdb_safe_append_history()':
/vol/src/gnu/gdb/gdb/local/gdb/top.c:1170:59: error: format '%d' expects argument of type 'int', but argument 3 has type 'pid_t {aka long int}' [-Werror=format=]
     = xstrprintf ("%s-gdb%d~", history_filename, getpid ());
                                                           ^

Fixed by casting pid_t to long and printing it as such.