Use stow for dependency management

This commit introduces the usage of GNU stow[1] for dependency
management.
Stow uses symlinks to make dependencies available in a single directory
althoguh they were installed in distinct directories.
Keeping installation directories seperate has the advantage of letting the
build fail if AM_CFLAGS and LDADD do not contain all dependencies which are
actually used.
Installing multiple dependencies into a single directory causes x_CFLAGS
and x_LIBS variables to magically point where other dependencies are
found as well, therefore missing entries can be overlooked.

Stow acts as a convenience layer here, making it unnecessary to supply a
list of locations in LD_LIBRARY_PATH, PKG_CONFIG_PATH and so forth for
building when dependencies are installed in distinct directories
manually.
Stow has to be present on the jenkins build nodes for successful executing of
osmo-build-dep.sh.

[1] https://www.gnu.org/software/stow/

Change-Id: I8f5012419495a656912b7b71e4f76ce102c6b63a

scripts/osmo-build-dep.sh

@@ -43,6 +43,14 @@ cd "$deps"
 osmo-deps.sh "$project" "$branch"
 cd "$project"
 
+# Keep the installation targets of the dependencies in a seperate directory
+# hierarchy before stowing them to avoid wrongly suggesting that they are part
+# of the -I and -L search paths
+mkdir -p "$inst/stow"
+
 autoreconf --install --force
-./configure --prefix="$inst" $cfg
+./configure --prefix="$inst/stow/$project" $cfg
 $MAKE $PARALLEL_MAKE install
+
+# Make the dependencies available through symlinks in $deps ($PWD/..).
+STOW_DIR="$inst/stow" stow --restow $project