Try to fix testsuite

When configuring tools/typelib a c-level define "TYPELIB_PLUGIN_PATH" is added to the compile-flags of "pluginmanager.cc" which hardcodes the _install_ folder of typelib by default [1].

This results in typelib (and its testsuite)  _only_ running against installed plugins. The testsuite  ignores locally compiled plugins inside the build-folder. Testing without contaminating the install-folder was by design impossible.

Now the hard-coded plug-lookup-path can be overridden by a environment variable TYPELIB_PLUGIN_PATH. Testing  without installing:

    TYPELIB_PLUGIN_PATH=`find build -name 'libtypeLang_*.so' -printf "%h:"` ./build/test/typelib_testsuite

this also works, reproducing the "old" behaviour:

    TYPELIB_PLUGIN_PATH=$ROCK_INSTALL_DIR/lib/typelib ./build/test/typelib_testsuite

But the env-var is not needed:

    ./build/test/typelib_testsuite

[1] https://github.com/orocos-toolchain/typelib/blob/master/typelib/CMakeLists.txt#L9