cmake: use full path for pkgconfig

CMakeLists.txt

@@ -178,8 +178,8 @@ include_directories (
 include (GNUInstallDirs)
 set (prefix ${CMAKE_INSTALL_PREFIX})
 set (exec_prefix "\${prefix}")
-set (libdir "\${exec_prefix}/${CMAKE_INSTALL_LIBDIR}")
-set (includedir "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
+set (libdir "${CMAKE_INSTALL_FULL_LIBDIR}")
+set (includedir "${CMAKE_INSTALL_FULL_INCLUDEDIR}")
 set (bindir "${CMAKE_INSTALL_FULL_BINDIR}")
 set (sysconfdir "${CMAKE_INSTALL_FULL_SYSCONFDIR}")
 set (top_srcdir "${CMAKE_CURRENT_SOURCE_DIR}")