build: mingw64 not supported

Before the release do porting to mingw64. Probably requires minor tweaking like adding mingw64 beside mingw32 read-time conditionals.

Probably it should be enough to fix the following files:

lsp/config.lsp.in
cmp/cmpmain.lsp
cmp/cmpos-features.lsp
aclocal.m4 # remember to call autoreconf -ivf and commit changes to configure script