This fixes issues when using LaTeX in doxygen

Previous attempt to work around the tine linking issue did not work.

This is a work around for not finding libtinemt at runtime. The DOOCS serverlib is lacking the RPATH, hence its dependencies are not found. If clientlib is linked first, tine is already linked.

Search explicitly for libgul14 which is required by DOOCS libraries

If a header-only library requires linking a 3rd party library, the ..._LIBRARIES variable could not be used in depending projects, because it
contained NOTFOUND.

Removed libtine linking workaround from FindDOOCS. Doocs libraries now are themselves linked correctly against tine.

Automatically adding CMAKE_CXX_STANDARD to all projects is giving too much trouble because different CMAKE versions add different --std=gnu... flags which do not play well the the --std=c++... which we use.

This reverts commit ddb4d575.

This confused git massively

The pkg-config file is missing so we have to search for the library
manually

This is a work-around for libDOOCSapi linking against it but expecting
/export/doocs/lib to be in the linker search path (not using rpath)

May be reverted once/if doocs fixes this

FindDOOCS: extend PKG_CONFIG_PATH by directory set via DOOCS_DIR (if set) and the hardcoded default in /export/doocs/...