pybind11.pc: use pcfiledir for relative destinations

If the datarootdir is absolute, just use the absolute path directly.
However, if it is relative, we can compute the prefix from the location
of the `.pc` file itself. This allows the install to be relocatable.

CMakeLists.txt

@@ -278,7 +278,16 @@ if(PYBIND11_INSTALL)
 
   # pkg-config support
   if(NOT prefix_for_pc_file)
-    set(prefix_for_pc_file "${CMAKE_INSTALL_PREFIX}")
+    if(IS_ABSOLUTE "${CMAKE_INSTALL_DATAROOTDIR}")
+      set(prefix_for_pc_file "${CMAKE_INSTALL_PREFIX}")
+    else()
+      set(pc_datarootdir "${CMAKE_INSTALL_DATAROOTDIR}")
+      set(prefix_for_pc_file "\${pcfiledir}/..")
+      while(pc_datarootdir)
+        get_filename_component(pc_datarootdir "${pc_datarootdir}" DIRECTORY)
+        string(APPEND prefix_for_pc_file "/..")
+      endwhile()
+    endif()
   endif()
   join_paths(includedir_for_pc_file "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
   configure_file("${CMAKE_CURRENT_SOURCE_DIR}/tools/pybind11.pc.in"