Make g2o headers relocatable by using build/install placeholders

[schweitzer]

Hello,

I've made some changes in the CMakeLists of g2o and I though it could be interesting to share it.

First I've set output directory to the BUILD_DIR instead of g2o_SOURCE_DIR.

Then I've updated most of the g2o modules headers to be relocatable (in a modern cmake way) by using placeholders. Before headers paths was hard-coded in the generated g2oTargets.cmake, and with this modification path are relatives to the install path.

Example with stuff module

before:

(g2oTragets.cmake)
...
 # Create imported target g2o::stuff
 add_library(g2o::stuff SHARED IMPORTED)
 
 set_target_properties(g2o::stuff PROPERTIES
   INTERFACE_INCLUDE_DIRECTORIES "/home/mschweitzer/dev/others/install/g2o/release/include"
 )

after:

(g2oTragets.cmake)
...
 # Create imported target g2o::stuff
 add_library(g2o::stuff SHARED IMPORTED)
 
 set_target_properties(g2o::stuff PROPERTIES
   INTERFACE_INCLUDE_DIRECTORIES "${_IMPORT_PREFIX}/include"
 )

the ${_IMPORT_PREFIX} is automatically generated by CMake and is relative to the install directory.

I think this is much more convenient to deal with transitive dependencies (when using for example ORB_SLAM2, OpenVSLAM, etc...).

I didn't check all the corner-case of g2o library, so let my know if there is anything wrong with this PR.