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

configure.ac

@@ -79,9 +79,6 @@ AC_PATH_TOOL(OBJCOPY, objcopy)
 
 AC_ARG_VAR(PYTHONPATH, Augments the default search path for python module files)
 
-dnl pkg-config check.
-PKG_PROG_PKG_CONFIG
-
 # Enable wallet
 AC_ARG_ENABLE([wallet],
   [AS_HELP_STRING([--disable-wallet],
@@ -375,6 +372,16 @@ case $host in
      ;;
 esac
 
+if test x$use_pkgconfig = xyes; then
+  m4_ifndef([PKG_PROG_PKG_CONFIG], [AC_MSG_ERROR(PKG_PROG_PKG_CONFIG macro not found. Please install pkg-config and re-run autogen.sh.)])
+  m4_ifdef([PKG_PROG_PKG_CONFIG], [
+  PKG_PROG_PKG_CONFIG
+  if test x"$PKG_CONFIG" = "x"; then
+    AC_MSG_ERROR(pkg-config not found.)
+  fi
+  ])
+fi
+
 if test x$use_comparison_tool != xno; then
   AC_SUBST(JAVA_COMPARISON_TOOL, $use_comparison_tool)
 fi
@@ -752,12 +759,7 @@ fi
 fi
 
 if test x$use_pkgconfig = xyes; then
-
-  if test x"$PKG_CONFIG" = "x"; then
-    AC_MSG_ERROR(pkg-config not found.)
-  fi
-
-  : #NOP
+  : dnl
   m4_ifdef(
     [PKG_CHECK_MODULES],
     [
@@ -1058,6 +1060,13 @@ AC_SUBST(TESTDEFS)
 AC_SUBST(LEVELDB_TARGET_FLAGS)
 AC_SUBST(MINIUPNPC_CPPFLAGS)
 AC_SUBST(MINIUPNPC_LIBS)
+AC_SUBST(CRYPTO_LIBS)
+AC_SUBST(SSL_LIBS)
+AC_SUBST(EVENT_LIBS)
+AC_SUBST(EVENT_PTHREADS_LIBS)
+AC_SUBST(ZMQ_LIBS)
+AC_SUBST(PROTOBUF_LIBS)
+AC_SUBST(QR_LIBS)
 AC_CONFIG_FILES([Makefile src/Makefile share/setup.nsi share/qt/Info.plist src/test/buildenv.py])
 AC_CONFIG_FILES([qa/pull-tester/run-bitcoind-for-test.sh],[chmod +x qa/pull-tester/run-bitcoind-for-test.sh])
 AC_CONFIG_FILES([qa/pull-tester/tests_config.py],[chmod +x qa/pull-tester/tests_config.py])