Fetch BSD regex from GitHub at build time

Replace bundled Spencer regex with a fetch-regex build step that
clones https://github.com/garyhouston/regex.git, generates the
required .ih and regex.h headers via mkh, and copies the result
into sys/amiga/regex/.

Usage: make CROSS_TO_AMIGA=1 fetch-regex

The fetched sources are not tracked in git.

.gitignore

@@ -96,3 +96,5 @@ bundle/*
 util/*.lib
 util/*.exp
 submodules/CHKSUMS.tmp
+sys/amiga/regex/
+sys/amiga/regex/