build: fix Windows builds without pkg-config

• guard PKG_PROG_PKG_CONFIG with an m4_ifdef. If not building for windows, require it
• add nops as necessary in case the ifdef reduces the if/then to nothing
• AC_SUBST some missing _LIBS. These were split out over time, but not all were properly substituted. They continued to work if pkg-config is installed because it does the AC_SUBST itself