Fix CFLAGS in configure regarding -O flag passing to SunCC compiler

* configure.ac (CFLAGS): Add -O flag if Solaris SunCC (i.e., not GCC)
regardless of threads support (i.e., even in case it is off).
Fix 'illegal option -xassembler-with-cpp' error (Oracle SunCC)

* Makefile.am (ASM_CPP_OPTIONS): Move comment to configure.ac.
* Makefile.am (COMPILER_XLC): Rename to ASM_WITH_CPP_UNSUPPORTED.
* configure.ac (COMPILER_XLC): Likewise.
* configure.ac (compiler_suncc): New variable (set to "yes" if Solaris
Sun C compiler).
* configure.ac (ASM_WITH_CPP_UNSUPPORTED): Define not only for
compiler_xlc but also for compiler_suncc.
Fix file descriptor resource leak in GC_register_data_segments (OS/2)

* os_dep.c (GC_register_data_segments): Call fclose(myexefile) on
return (only if OS2).
Fix 'variable unused' warning in GC_save_callers

* os_dep.c (GC_save_callers): Do not declare "i" local variable unless
it is used.
Fix signedness of char values passed to isspace, iscntrl, isxdigit
(eliminate 'array subscript has type char' GCC warning)

* cord/tests/de_win.c (WinMain, plain_chars, control_chars): Cast char
value to unsigned char when passed to isspace(), iscntrl().
* os_dep.c (GC_parse_map_entry): Change type of "start_start",
"end_start", "maj_dev_start", "p" local variables from char* to
unsgined char* (as isspace(), isxdigit() expect non-negative values);
insert casts between char* and unsigned char* where needed.
* os_dep.c (GC_parse_map_entry): Remove "endp" local variable (use "p"
variable directly instead).