Added support for "no-parallel-build" option

This commit adds support for forcing a buildfile to be built in
non-parallel by introducing a new options list and a special toggle
option named "no-parallel-build".

To force a buildfile to be built in non-parallel, simply add to
buildfile:

options=no-parallel-build

This is reported to be useful for eg. kernel module builds which must
avoid to be built at the same time because they operate in the same
kernel build workspace.

In the future, more options might be supported in the options list.
Merge branch 'no-parallel-build-option' of github.com:buildgear/buildgear into no-parallel-build-option

Conflicts:
	src/buildmanager.cc