pkg-config: add Requires.private field to allow static builds

[lovell]

This ensures the linker is made aware of symbols from transitive dependencies.

[lovell]

It looks like the libde265 (2) CI tasks install libde265 to a non-standard location but don't update PKG_CONFIG_PATH accordingly, hence the failure of the go / libde265 (2) task.

I think the following CI change might get it passing, but unsure if this is the best approach.

diff --git a/scripts/prepare-ci.sh b/scripts/prepare-ci.sh
index c1a729d..3f9ba15 100755
--- a/scripts/prepare-ci.sh
+++ b/scripts/prepare-ci.sh
@@ -63,6 +63,7 @@ if [ -z "$CHECK_LICENSES" ] && [ -z "$CPPLINT" ] && [ -z "$CMAKE" ]; then
     fi
     if [ ! -z "$GO" ]; then
         CONFIGURE_ARGS="$CONFIGURE_ARGS --prefix=$BUILD_ROOT/dist --disable-gdk-pixbuf"
+        export PKG_CONFIG_PATH="$PKG_CONFIG_PATH:$BUILD_ROOT/dist"
     else
         CONFIGURE_ARGS="$CONFIGURE_ARGS --disable-go"
     fi

UPDATE: It looks like PKG_CONFIG_PATH is configured in a few places and I've now modified enough of them to keep the CI tests happy. Perhaps consolidation of these could be a useful improvement, but that's probably for a separate PR.

[lovell]

(Apologies for the CI noise, I'll hopefully work out the relevant PKG_CONFIG_PATH to modify soon.)

[farindk]

Thank you. I resolved the merge conflicts. Please check that your changes are still working. I'll then merge it into master.

[lovell]

@farindk LGTM, thank you.

[farindk]

Good. Has been merged.