Support building selected C/C++ files in msvs-ninja

This commits adds a new 'msvs_external_builder_clcompile_cmd' property, which
will be used when the user tries building selected C/C++ files.

For msvs-ninja, the 'msvs_external_builder_clcompile_cmd' will be mapped to a
new 'ExecClCompile' command in gyp-win-tool, which will expand the list of
selected files into a list of ninja targets and then invoke ninja.

BUG=352876
R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/203383003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1878 78cadc50-ecff-11dd-a971-7dbc132099af