Problem: GNU extensions, such as `ifeq` and `wildcard` function, are
highlighted in BSDmakefile
Solution: detect BSD, GNU, or Microsoft implementation according to
filename, user-defined global variables, or file contents
closes: vim/vim#17089f35bd76b31
Co-authored-by: Eisuke Kawashima <e-kwsm@users.noreply.github.com>
Co-authored-by: Roland Hieber <rohieb@users.noreply.github.com>