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

@@ -75,9 +75,6 @@ AC_PATH_PROG(HEXDUMP,hexdump)
 AC_PATH_TOOL(READELF,readelf)
 AC_PATH_TOOL(CPPFILT,c++filt)
 
-dnl pkg-config check.
-PKG_PROG_PKG_CONFIG
-
 # Enable wallet
 AC_ARG_ENABLE([wallet],
   [AS_HELP_STRING([--enable-wallet],
@@ -345,6 +342,16 @@ 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$use_comparison_tool != xno; then
   if test x$JAVA = x; then
     AC_MSG_ERROR("comparison tool set but java not found")
@@ -619,12 +626,7 @@ BOOST_LIBS="$BOOST_LDFLAGS $BOOST_SYSTEM_LIB $BOOST_FILESYSTEM_LIB $BOOST_PROGRA
 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],
     [
@@ -861,6 +863,11 @@ AC_SUBST(LIBTOOL_APP_LDFLAGS)
 AC_SUBST(BOOST_LIBS)
 AC_SUBST(TESTDEFS)
 AC_SUBST(LEVELDB_TARGET_FLAGS)
+AC_SUBST(CRYPTO_LIBS)
+AC_SUBST(SSL_LIBS)
+AC_SUBST(EVENT_LIBS)
+AC_SUBST(EVENT_PTHREADS_LIBS)
+AC_SUBST(ZMQ_LIBS)
 AC_SUBST(GMP_LIBS)
 AC_SUBST(GMPXX_LIBS)
 AC_SUBST(LIBSNARK_DEPINST)