Add API version to everything that gets installed

[mcatanzaro]

We should add the API version to:

• include directory path
• installed library
• pkg-config files

etc.

[mcatanzaro]

I noticed this problem looking at my installed headers. WPE WebKit installs into /usr/include/wpe-0.1. WPEBackend installs into /usr/include/wpe.

One solution would be to put them both in wpe-0.1 and expect the API versions to be synced. But this means it will be almost impossible to ever change the WPEBackend API. It would surely be better for WPEBackend to not share an API version with WPE WebKit. This means we should either:

• Come up with a different API name for WPEBackend, e.g. "wpe-backend-0.1" instead of "wpe"; or
• Install WPE WebKit's headers differently, e.g. under wpe-webkit-0.1 instead of wpe-0.1

The later solution seems nicer to me.

[mcatanzaro]

Also, I notice this:

set_target_properties(WPEBackend PROPERTIES VERSION ${WPE_VERSION} SOVERSION ${WPE_VERSION_MAJOR})

which is maybe OK, but means you can't make any ABI changes without bumping the major version. If that's intended, then OK.

[mcatanzaro]

WPE WebKit installs into /usr/include/wpe-0.1. WPEBackend installs into /usr/include/wpe.

Well, currently WPE WebKit uses /usr/include/wpe-0.1/WPE, but I remove the last WPE part in this bug and then we wind up with this conflict. My suggestion would be to use /usr/include/wpe-webkit-0.1/ instead.