* configure.in: Recognize --with-headers flag and determine
commandline options to be used in compilation.
* Makeconfig (+includes): Use $(sysincludes).
* config.make.in (sysincludes): Define from SYSINCLUDES.
* glibcbug.in: Use SYSINCLUDES information.
Patches by Zack Weinberg.
* sysdeps/sparc/sparc32/elf/start.S: Rewrite for __libc_start_main.
* sysdeps/sparc/sparc64/elf/start.S: Likewise.