This is a big change: apart from removing LLVM 14 it also removes typed
pointer support (which was only fully supported in LLVM up to version
14). This removes about 200 lines of code, but more importantly removes
a ton of special cases for LLVM 14.
Prior to this commit, the build instructions were encoded in Makefiles
that contained GNU extensions that are unique to GNU Make and
incompatible with older implementations.
Thie commit renames all Makefiles to GNUmakefile which clearly denotes
that the file contains GNU extensions.
GNU Make actually first looks for a GNUmakefile, then makefile, THEN
Makefile so in addition to simply being more correct and portable, it
saves a few unnecessary failed attempts to open the correct build
file.
