Fix install directory, address #116

[wxmerkt]

The issue underlying #116 is that:

1. The library gets installed to ${CMAKE_INSTALL_PREFIX}/lib
2. However, the use of GNUInstallDirs inside the cmake/base.cmake sets the variable defining what gets set in the pkg-config template to use the FULL variables. These get set to include the prefix on some platforms (and old versions of debhelper).
3. As a result there is a mismatch between what the eigenpy.pc says for where the library and pkg-config files are and where they actually get installed.

This took quite some time to debug...

[wxmerkt]

@nim65s Will this be fine for you guys? I reckon this issue may lurk in other cmakemodules depending projects as well...

[jcarpent]

This is fine for me. I think @seanyen won't be affected by these changes and neither for LAAS side.
I will merge it and may create a new release.
@wxmerkt Is it OK for you?

[jcarpent]

@wxmerkt Thanks a lot for fixing this annoying bug.

[nim65s]

If it fixes the build for you, and if that's the right way to do it, then it's ok for me.
For robotpkg, I don't think it will change something, and if it does, I will be able to override it anyway :)

[nim65s]

(i would have preferred shorter things like lib to ${CMAKE_INSTALL_FULL_INCLUDEDIR}, but this is not relevant for now.)

[jcarpent]

@nim65s Thanks for your feedback.

[wxmerkt]

Thank you guys! @jcarpent Can you please increment the package.xml and make a new release? :-)