This version takes to add the include path to the target definition for
cmake 2.8.11 and later.  Also the documentation sticks to the existing
convention of using cmake variables ${PROJ4_LIBRARIES} and
${PROJ4_INCLUDE_DIRS}.  However, the namespace variables are still being
included.

Here's the roll-out plan

(0) Version 4.9.x: The target is proj and PROJ4_LIBRARIES is set to
this.

(1) Version 5.0.x: Two targets, proj and PROJ4::proj, are defined;
PROJ4_LIBRARIES = proj.

(2) In a year or two: Two targets, proj and PROJ4::proj, are defined;
PROJ4_LIBRARIES = PROJ4::proj.

(3) With a change in the library which breaks backwards compatibility:
The target is PROJ4::proj and PROJ4_LIBRARIES = PROJ4::proj.

* install proj4 data in share/proj, not share/

* no need for CMP0022 policy anymore

* fix #560 install proj4-config.cmake to defaut path

* add some variable and cmake option to specify where
the cmake config file should be installed.
* change project_root_dir use in project-config.cmake.in
variable from hard coded path to computed path with
file(RELATIVE_PATH) function.
* Still use the old project-config.cmake/configure_file way
of generate config file. A cleaner way could be to use
the modern cmake export(target ..)

* use GNUInstallDirs for default unix install path

These files are generated by automake.