Figure out how to support gnustep-make's support for Additional files

[qmfrederik]

In gnustep-make, libraries can deploy additional files to share/GNUstep/Makefiles/Additional/. gnustep-base does this to set additional preprocessor variables. Because Conan deploys each package into its own directory, these Additional files end up being split in multiple locations. This causes a build failure in gui-libs, e.g. because GNUSTEP_BASE_HAVE_LIBXML is not set.

[qmfrederik]

• In the install phase, the full path to the gnustep-make directory gets pulled in, we'll have to manually account for that
• For dependencies (libs-gui), we can probably set GNUSTEP_LOCAL_ADDITIONAL_MAKEFILES...
• ... although libs-gui itself defines that var, perhaps we need another var which tools-make reads from (as a patch?)