[build/install] Fix Eigen install path issue. openMVG#1335 (openMVG#1117

, openMVG#1118)

Since there is a bug in Eigen cmake file with the usage of
`INCLUDE_INSTALL_DIR` and `EIGEN_INCLUDE_INSTALL_DIR` we force here the
usage of `EIGEN_INCLUDE_INSTALL_DIR`. It avoids the definition of
`${CMAKE_INSTALL_INCLUDEDIR}/eigen3` by default that will go to
`/usr/include` even we use a local specified directory.

src/third_party/CMakeLists.txt

@@ -92,7 +92,7 @@ add_subdirectory(fast)
 ##
 if (DEFINED OpenMVG_USE_INTERNAL_EIGEN)
   #Configure Eigen install
-  set(INCLUDE_INSTALL_DIR ${CMAKE_INSTALL_PREFIX}/include/openMVG/third_party/eigen)
+  set(EIGEN_INCLUDE_INSTALL_DIR ${CMAKE_INSTALL_PREFIX}/include/openMVG/third_party/eigen)
   add_subdirectory(eigen)
 endif()
 

src/third_party/eigen/CMakeLists.txt

@@ -64,10 +64,7 @@ include_directories(${CMAKE_CURRENT_SOURCE_DIR} ${CMAKE_CURRENT_BINARY_DIR})
 
 # Backward compatibility support for EIGEN_INCLUDE_INSTALL_DIR
 if(EIGEN_INCLUDE_INSTALL_DIR)
-  message(WARNING "EIGEN_INCLUDE_INSTALL_DIR is deprecated. Use INCLUDE_INSTALL_DIR instead.")
-endif()
-
-if(EIGEN_INCLUDE_INSTALL_DIR AND NOT INCLUDE_INSTALL_DIR)
+  #message(WARNING "EIGEN_INCLUDE_INSTALL_DIR is deprecated. Use INCLUDE_INSTALL_DIR instead.")
   set(INCLUDE_INSTALL_DIR ${EIGEN_INCLUDE_INSTALL_DIR}
       CACHE PATH "The directory relative to CMAKE_PREFIX_PATH where Eigen header files are installed")
 else()