Allow overriding gcc with the CC env var in const_generator.rb and struct_generator.rb

[leshik]

Right now one can override gcc for types_generator.rb, but not const_generator.rb and struct_generator.rb. This PR fixes this.

Related: typhoeus/ethon#186

[larskanis]

There could be more compiler independence in this part of the code, probably.

It also makes the compiler a runtime dependency of the gem, which most ffi based gems don't have otherwise. That's why these classes are rarely used. On the other hand ffi.gem needs a compiler anyway in most cases.

[leshik]

On the other hand ffi.gem needs a compiler anyway in most cases.

@larskanis Right, but it's usually a good idea to separate build & runtime. E.g. in my case, gcc is used on bundle install stage to build things like nokogiri and ffi, but it's missing at runtime. Now I added tcc to runtime as it adds only 100kb overhead to the Docker image.

[Baltazardoung]

Can you tell us when a new version (v1.15.1 ?) including this patch will be released ? Thanks a lot.