Actually use CFLAGS to build vendor libs

Ran into some warnings here because CPPFLAGS were untouched, but the CFLAGS environment variable is overridden at Rakefile:149.

Also got confused because blueprint.rb sets up cflags everywhere, but they don't have any effect when you use #command to call configure or make.

Manually propagate the CFLAGS to the vendor libs. Would have used Shellwords instead of #inspect, but I don't know how well that works on Windows. Seems brittle either way.