CMakePackageConfigHelpers: allow to specify a relative CMAKE_INSTALL_PREFIX

Modules/CMakePackageConfigHelpers.cmake

@@ -241,8 +241,10 @@ function(CONFIGURE_PACKAGE_CONFIG_FILE _inputFile _outputFile)
     else()
       message(FATAL_ERROR "INSTALL_PREFIX must be an absolute path")
     endif()
-  else()
+  elseif(IS_ABSOLUTE "${CMAKE_INSTALL_PREFIX}")
     set(installPrefix "${CMAKE_INSTALL_PREFIX}")
+  else()
+    get_filename_component(installPrefix "${CMAKE_INSTALL_PREFIX}" ABSOLUTE)
   endif()
 
   if(IS_ABSOLUTE "${CCF_INSTALL_DESTINATION}")