Support custom build
I want to use [libnice](https://libnice.freedesktop.org) from Nim and tried to wrap it with `getHeader` macro but I got error because libnice uses neither autoconf nor cmake.
libnice uses only [Meson Build System](https://mesonbuild.com).
I think allowing user to write custom build code would be better than adding Meson build system support to Nimterop because meson is not widely used like autoconf or cmake and we can still build C libraries even if they use neither autoconf nor cmake.

How about to add `btCustom` to `BuildType` enum type and let user implement `customBuildLibrary` proc that build the library and `getHeader` macro calls it when `btCustom` is specified to `buildTypes` parameter?
Or add a new macro `getHeaderCustomBuild` that works like `getHeader` but requires a proc that build the library?