[SV 63516] [DOS] Support include files with drivespecs

[SV 63516] [DOS] Support include files with drivespecs

* src/makeint.h (HAVE_DRIVESPEC): Create a macro to check.
* src/main.c (.FEATURES): Add "dospaths" as a feature.
* src/read.c (eval_makefile) [DOS]: If the included makefile name
starts with a drivespec, don't search the include directories.
* doc/make.texi (Include): Document this behavior.
* tests/scripts/features/include: Add a test.