fcm make: build: allow separate linker

`build.prop{ld}` can now be used to specify a linker that is not the compiler.
`build.prop{keep-lib-o}=true` can now be used to keep the lib*.a file that
contains the dependent objects of the executable.
fcm make: build: document new link props
Merge branch 'master' into add-fcm-make-build-link-props
fcm make: build: tests for new link props

Tests added for `build.prop{ld}` and `build.prop{keep-lib-o}`.
Merge pull request #20 from matthewrmshin/add-fcm-make-build-link-props

fcm make: build: allow separate linker