Ninja: job pool support for compiling and linking

Could be tested by setting the environment
variable NINJA_STATUS=[%r]
Help: Fix typo in genex docs.

would be require -> would require
Help: Fix typo in CMP0040 docs.

'report and error' -> 'report an error'
Merge topic 'minor-cleanups'

ebeb31d Help: Fix typo in CMP0040 docs.
97b0abb Help: Fix typo in genex docs.
Merge topic 'ninja-compile-link-pool'

7605e37 Ninja: job pool support for compiling and linking