make gdbserver use the same ptrace autoconf checks as gdb

make gdbserver use the same ptrace autoconf checks as gdb

This factors the ptrace checks out of gdb's configure.ac to a new
ptrace.m4 file, and then makes gdbserver's configure.ac source it too.

gdb/ChangeLog:
2015-07-24  Pedro Alves  <palves@redhat.com>

	* acinclude.m4: Include ptrace.m4.
	* configure.ac: Call GDB_AC_PTRACE and move ptrace checks ...
	* ptrace.m4: ... to this new file.

gdb/gdbserver/ChangeLog:
2015-07-24  Pedro Alves  <palves@redhat.com>

	* acinclude.m4: Include ../ptrace.m4.
	* configure.ac: Call GDB_AC_PTRACE.
	* config.in, configure: Regenerate.