ninja/mac: Only write pch flag variables if needed.
ninja/mac: Only write pch flag variables if needed.

No intended functionality change:
These variables are empty in the majority of targets, they aren't defined in
the toplevel ninja file, and ninja expands undefined variables as empty strings.
Reduces ninja file line count by ~3000 for chromium (of 148000, so it's
only a 2% reduction).

Likewise, only write libtool_flags if they are not empty (they're _always_
empty in Chromium for example). Reduces ninja file line count by number of
static library targets (~hundreds).

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/18808003
witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp,witwall/gyp