Allow projects to be removed post inclusion

Suppose I have a makefile like:

core = 6.x
projects[views] = 2.11
projects[strongarm] = 2.0

And it is included by another makefile like this:

includes[] = http://example.com/foo.make

; ... other stuff

While the second makefile can override properties of the first, there is no way to remove a particular entry of projects[] without redefining the projects array entirely from the start.

This patch allows for the following to be valid:

includes[] = http://example.com/foo.make

projects[views] = FALSE

This is valid .info file syntax (sets $projects['views'] = FALSE) and would mean to omit / unset this entry altogether. It can of course be overridden by later lines or another includer.