gdb: disable python when python is not available.

The patch checkes whether the python executable is available
at runtime. If it is not available, it disables python at run_time.

This backports a local patch from ChromeOS. The bug entry for
the ChromeOS is at crbug.com/248605

This CL also check some pre defines to include unistd.h and call
the function fork().

BUG: crbug.com/248605
5 files changed
tree: b0ec5ce5a2182c68f5ab3a6baca56ed7f946fb5f
  1. gdb-7.10/
  2. build-gdb.sh
  3. build.py