Fix CMake directory issues.

Allows absolute and relative paths for
- `INCLUDE_INSTALL_DIR`
- `CMAKEPACKAGE_INSTALL_DIR`
- `PKGCONFIG_INSTALL_DIR`

Type should be `PATH` not `STRING`.  Contrary to !211, these don't
seem to be made absolute if user-defined - according to the doc any
directories should use `PATH` type, which allows a file dialog
to be used via the GUI.  It also better handles file separators.

If user provides an absolute path, it will be made relative to
`CMAKE_INSTALL_PREFIX` so that the `configure_packet_config_file` will
work.

Fixes #2155 and #2269.

1 files changed, 6 insertions, 5 deletions

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 7ba7d3872..bd1af32b2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -424,25 +424,26 @@ endif()
 
 if(EIGEN_INCLUDE_INSTALL_DIR AND NOT INCLUDE_INSTALL_DIR)
   set(INCLUDE_INSTALL_DIR ${EIGEN_INCLUDE_INSTALL_DIR}
-      CACHE STRING "The directory relative to CMAKE_PREFIX_PATH where Eigen header files are installed")
+      CACHE PATH "The directory relative to CMAKE_INSTALL_PREFIX where Eigen header files are installed")
 else()
   set(INCLUDE_INSTALL_DIR
       "${CMAKE_INSTALL_INCLUDEDIR}/eigen3"
-      CACHE STRING "The directory relative to CMAKE_PREFIX_PATH where Eigen header files are installed"
+      CACHE PATH "The directory relative to CMAKE_INSTALL_PREFIX where Eigen header files are installed"
       )
 endif()
 set(CMAKEPACKAGE_INSTALL_DIR
     "${CMAKE_INSTALL_DATADIR}/eigen3/cmake"
-    CACHE STRING "The directory relative to CMAKE_PREFIX_PATH where Eigen3Config.cmake is installed"
+    CACHE PATH "The directory relative to CMAKE_INSTALL_PREFIX where Eigen3Config.cmake is installed"
     )
 set(PKGCONFIG_INSTALL_DIR
     "${CMAKE_INSTALL_DATADIR}/pkgconfig"
-    CACHE STRING "The directory relative to CMAKE_PREFIX_PATH where eigen3.pc is installed"
+    CACHE PATH "The directory relative to CMAKE_INSTALL_PREFIX where eigen3.pc is installed"
     )
 
 foreach(var INCLUDE_INSTALL_DIR CMAKEPACKAGE_INSTALL_DIR PKGCONFIG_INSTALL_DIR)
+  # If an absolute path is specified, make it relative to "{CMAKE_INSTALL_PREFIX}".
   if(IS_ABSOLUTE "${${var}}")
-    message(FATAL_ERROR "${var} must be relative to CMAKE_PREFIX_PATH. Got: ${${var}}")
+    file(RELATIVE_PATH "${var}" "${CMAKE_INSTALL_PREFIX}" "${${var}}")
   endif()
 endforeach()