meson: Replace 'config.h' with config_file

Scott Anderson requested to merge ascent/drm:config-path into master

This fixes an issue with libdrm failing to build when used as a meson subproject. Using 'config.h' directly will cause it to possibly refer to the wrong file.

By using @0@.format(config_file), it will be transformed into the correct relative path, e.g. ./config.h in normal build, ./subprojects/libdrm/config.h in subproject build.

Merge request reports