Merge #18051: build: Fix behavior when ALLOW_HOST_PACKAGES unset

0e519fe build: Fix behavior when ALLOW_HOST_PACKAGES unset (Hennadii Stepanov)

Pull request description:

  On master (f05c1ac) during building with depends host packages are always considered by `pkg-config` regardless of `ALLOW_HOST_PACKAGES` environment variable. This causes issues like #18042.

  This is an alternative to #18042 and #18045.

  On master:
  ```
  $ make HOST=x86_64-apple-darwin16 -C depends
  $ CONFIG_SITE=$PWD/depends/x86_64-apple-darwin16/share/config.site ./configure
  ...
  checking for QT_DBUS... yes
  ...
  checking whether to build GUI with support for D-Bus... yes
  ...
  ```

  ---

  With this PR:
  1) `ALLOW_HOST_PACKAGES` unset
  ```
  $ make HOST=x86_64-apple-darwin16 -C depends
  $ CONFIG_SITE=$PWD/depends/x86_64-apple-darwin16/share/config.site ./configure
  ...
  checking for QT_DBUS... no
  ...
  checking whether to build GUI with support for D-Bus... no
  ...
  ```
  2) `ALLOW_HOST_PACKAGES=1`
  ```
  $ make HOST=x86_64-apple-darwin16 ALLOW_HOST_PACKAGES=1 -C depends
  $ CONFIG_SITE=$PWD/depends/x86_64-apple-darwin16/share/config.site ./configure
  ...
  checking for QT_DBUS... yes
  ...
  checking whether to build GUI with support for D-Bus... yes
  ...
  ```

ACKs for top commit:
  jonasschnelli:
    Tested ACK 0e519fe - After this PR (and after a `make clean` & `make HOST=x86_64-apple-darwin16` in depends), the macOS depends build on a Debian machine with qt as host package worked (it fails on master due to `ENABLE_DBUS=1`).

Tree-SHA512: d11e1c2bc8ce8a07f3f9b465b01c9b2c814afe75b085a8b88aab74fd3a922aa98180c447457dfc4174515513181c5f4edc521978a1d3d0a112106c98b5c73c0e

Author: laanwj

depends/config.site.in

@@ -59,7 +59,7 @@ PKG_CONFIG="`which pkg-config` --static"
 # avoid ruining the cache. Sigh.
 export PKG_CONFIG_PATH=$depends_prefix/share/pkgconfig:$depends_prefix/lib/pkgconfig
 if test -z "@allow_host_packages@"; then
-  export PKGCONFIG_LIBDIR=
+  export PKG_CONFIG_LIBDIR=$depends_prefix/lib/pkgconfig
 fi
 
 CPPFLAGS="-I$depends_prefix/include/ $CPPFLAGS"