gen_makefile: don't shell out to nproc

Use python's multiprocessing.cpu_count() instead of "$(nproc)". The
latter didn't work properly in Makefiles, the right syntax would have
been "$(shell nproc)". Make didn't complain about it and assumed that we
want to use all CPUs with an empty argument "-j ", but meson doesn't
accept this syntax.

Change-Id: I58ca082339f3aff813f587f4c2be9c0951b9b2dd
---
M gen_makefile.py
1 file changed, 19 insertions(+), 1 deletion(-)

diff --git a/gen_makefile.py b/gen_makefile.py
index 9a8a5cf..7fc2bd2 100755
--- a/gen_makefile.py
+++ b/gen_makefile.py
@@ -49,6 +49,7 @@
 import sys
 import os
 import argparse
+import multiprocessing
 
 topdir = os.path.dirname(os.path.realpath(__file__))
 all_deps_file = os.path.join(topdir, "all.deps")
@@ -85,7 +86,9 @@
 parser.add_argument('-o', '--output', dest='output', default='Makefile',
   help='''Makefile filename (default: 'Makefile').''')
 
-parser.add_argument('-j', '--jobs', dest='jobs', default='$(nproc)', nargs='?', const='$(nproc)',
+parser.add_argument('-j', '--jobs', dest='jobs', type=int,
+  default=multiprocessing.cpu_count(), nargs='?',
+  const=multiprocessing.cpu_count(),
   help='''-j option to pass to 'make'.''')
 
 parser.add_argument('-I', '--sudo-make-install', dest='sudo_make_install',

