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

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