build: quit abusing LIBS for Windows builds.

Similar to the INCLUDES changes in 6b099402b40, split out LIBS into individual
entries for more fine-grained control.

Also add MINIUPNPC_LIBS which was missing before, and hook it up to
executables.
build: fix libtool's refusal to link static libs into a dll
Merge pull request #5086

1723862 build: fix libtool's refusal to link static libs into a dll (Cory Fields)
28d412f build: quit abusing LIBS for Windows builds. (Cory Fields)