build: fix Windows builds without pkg-config
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

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