Compiling C extensions in dep_selector Ruby gem with Rake on FreeBSD

I am trying to install Ruby gem: dep_gecode. It compiles some C extensions, but the compilation fails because the cpp compiler isn't run with properly set include folders.

When I run rake install it automatically creates a Makefile from extconf.rb to compile the extensions. When I edit the Makefile and manually add flag -I/usr/local/include the compilation of the extensions succeeds. But when I re-run rake install the Makefile gets overwritten and compilation fails again. Obviously I need to fix extconf.rb but I don't quite know how.

Now a bit more details. The source for dep_gecode gem is here:

https://github.com/opscode/dep-selector

The extconf.rb file that needs to be changed is here:

https://github.com/opscode/dep-selector/blob/master/ext/dep_gecode/extconf.rb

What I want to change in the generated Makefile is this line:

CPPFLAGS =  -I/usr/include   -I/usr/include -I/usr/local/include

instead of this one, which is being generated at this moment:

CPPFLAGS =  -I开发者_JS百科/usr/include   -I/usr/include

As you see, the include path /usr/local/include is missing. This is a standard include path on FreeBSD systems, which suggests that the problem may be with Rake, not with this particular extconf.rb file. Any help greatly appreciated.

Edit: I've just found a workaround. It is enough to add this line to extconf.rb to have that folder properly appearing in the generated Makefile:

$CPPFLAGS << "-I/usr/local/include"

Not sure if this would be the right way of dealing with this problem though...