Pass $(CC) as make parameter, not environment variable

[bstaletic]

Passing $(CC) as env variable is not effective when Makefile overrides $(CC).

If $(CC) is passed as parameter instead, make will override the setting from Makefile, which would allow YCM-Generator to be used in a greater number of projects.

So, make resolves variables according to the following priority order:

• Parameters passed to make
• Variables set inside Makefile itself
• Environment variables

The same goes for all other variables that YCM-Generator uses, not just $(CC).