build: Do not export `PKG_CONFIG_{PATH|LIBDIR}` variables

configure.ac

@@ -19,6 +19,16 @@ if test "$PKG_CONFIG" = ""; then
   AC_MSG_ERROR([pkg-config not found])
 fi
 
+# When compiling with depends, the `PKG_CONFIG_PATH` and `PKG_CONFIG_LIBDIR` variables,
+# being set in a `config.site` file, are not exported to let the `--config-cache` option
+# work properly.
+if test -n "$PKG_CONFIG_PATH"; then
+  PKG_CONFIG="env PKG_CONFIG_PATH=$PKG_CONFIG_PATH $PKG_CONFIG"
+fi
+if test -n "$PKG_CONFIG_LIBDIR"; then
+  PKG_CONFIG="env PKG_CONFIG_LIBDIR=$PKG_CONFIG_LIBDIR $PKG_CONFIG"
+fi
+
 BITCOIN_DAEMON_NAME=bitcoind
 BITCOIN_GUI_NAME=bitcoin-qt
 BITCOIN_CLI_NAME=bitcoin-cli

depends/config.site.in

@@ -84,12 +84,9 @@ fi
 
 PKG_CONFIG="$(which pkg-config) --static"
 
-# These two need to remain exported because pkg-config does not see them
-# otherwise. That means they must be unexported at the end of configure.ac to
-# avoid ruining the cache. Sigh.
-export PKG_CONFIG_PATH="${depends_prefix}/share/pkgconfig:${depends_prefix}/lib/pkgconfig"
+PKG_CONFIG_PATH="${depends_prefix}/share/pkgconfig:${depends_prefix}/lib/pkgconfig"
 if test -z "@allow_host_packages@"; then
-  export PKG_CONFIG_LIBDIR="${depends_prefix}/lib/pkgconfig"
+  PKG_CONFIG_LIBDIR="${depends_prefix}/lib/pkgconfig"
 fi
 
 CPPFLAGS="-I${depends_prefix}/include/ ${CPPFLAGS}"