Support command wrapper in make_global_settings

If $cmd_wrapper is defined in make_global_settings, it will be
prepend to $cmd.  For example,
 'make_global_settings': [
   ['CC_wrapper', '/usr/bin/distcc'],
   ['CC', '/usr/bin/gcc'],
 ]
then
 CC ?= /usr/bin/distcc /usr/bin/gcc  # Makefile
or
 cc = /usr/bin/distcc /usr/bin/gcc   # build.ninja

LINK_wrapper is pre defined as 'flock $(builddir)/linker.lock'

BUG=chromium:173686
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/12300015

git-svn-id: http://gyp.googlecode.com/svn/trunk@1586 78cadc50-ecff-11dd-a971-7dbc132099af