build: change handling of Makefile variables
build: change handling of Makefile variables

First, fix the problem that "ar" was being used for cross compilation,
instead of "i686-w64-mingw32-ar".  That happened because make itself
defines AR.

Second, use the normal makefile assignment syntax in more places.  I had
misunderstood makefile variable assignments.  It turns out that if you
set a makefile variable on a command line, then that will override
normal assignments.  That fact made most uses of the "?=" incorrectly
justified and unnecessary.

brainsickcc/libvbstd