build: Drop ALLOW_HOST_PACKAGES support in depends

The `ALLOW_HOST_PACKAGES` variable was introduced in bitcoin#10508 "to
speed up build and avoid timeout".

It is no longer the case for our CI infrastructure, which uses self-
hosted persistent workers and depends caching.

In the current circumstances, it does not seem worth porting this
feature to the upcoming CMake-based build system.

depends/Makefile

@@ -242,7 +242,6 @@ $(host_prefix)/share/config.site : config.site.in $(host_prefix)/.stamp_$(final_
             -e 's|@CXXFLAGS@|$(strip $(host_CXXFLAGS) $(host_$(release_type)_CXXFLAGS))|' \
             -e 's|@CPPFLAGS@|$(strip $(host_CPPFLAGS) $(host_$(release_type)_CPPFLAGS))|' \
             -e 's|@LDFLAGS@|$(strip $(host_LDFLAGS) $(host_$(release_type)_LDFLAGS))|' \
-            -e 's|@allow_host_packages@|$(ALLOW_HOST_PACKAGES)|' \
             -e 's|@no_qt@|$(NO_QT)|' \
             -e 's|@no_qr@|$(NO_QR)|' \
             -e 's|@no_zmq@|$(NO_ZMQ)|' \

depends/README.md

@@ -110,9 +110,6 @@ The following can be set when running make: `make FOO=bar`
 - `NO_UPNP`: Don't download/build/cache packages needed for enabling UPnP
 - `NO_NATPMP`: Don't download/build/cache packages needed for enabling NAT-PMP
 - `NO_USDT`: Don't download/build/cache packages needed for enabling USDT tracepoints
-- `ALLOW_HOST_PACKAGES`: Packages that are missed in dependencies (due to `NO_*` option or
-  build script logic) are searched for among the host system packages using
-  `pkg-config`. It allows building with packages of other (newer) versions
 - `MULTIPROCESS`: Build libmultiprocess (experimental, requires CMake)
 - `DEBUG`: Disable some optimizations and enable more runtime checking
 - `HOST_ID_SALT`: Optional salt to use when generating host package ids

depends/config.site.in

@@ -89,9 +89,7 @@ fi
 PKG_CONFIG="$(which pkg-config) --static"
 
 PKG_CONFIG_PATH="${depends_prefix}/share/pkgconfig:${depends_prefix}/lib/pkgconfig"
-if test -z "@allow_host_packages@"; then
-  PKG_CONFIG_LIBDIR="${depends_prefix}/lib/pkgconfig"
-fi
+PKG_CONFIG_LIBDIR="${depends_prefix}/lib/pkgconfig"
 
 CPPFLAGS="-I${depends_prefix}/include/ ${CPPFLAGS}"
 LDFLAGS="-L${depends_prefix}/lib ${LDFLAGS}"