Use CMAKE_INSTALL_FULL_{LIBDIR,INCLUDEDIR}. (#963)

diff --git a/CMake/AbseilHelpers.cmake b/CMake/AbseilHelpers.cmake
index 1a80b5b..1fa57a7 100644
--- a/CMake/AbseilHelpers.cmake
+++ b/CMake/AbseilHelpers.cmake
@@ -171,8 +171,8 @@
       FILE(GENERATE OUTPUT "${CMAKE_BINARY_DIR}/lib/pkgconfig/absl_${_NAME}.pc" CONTENT "\
 prefix=${CMAKE_INSTALL_PREFIX}\n\
 exec_prefix=\${prefix}\n\
-libdir=\${prefix}/${CMAKE_INSTALL_LIBDIR}\n\
-includedir=\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}\n\
+libdir=${CMAKE_INSTALL_FULL_LIBDIR}\n\
+includedir=${CMAKE_INSTALL_FULL_INCLUDEDIR}\n\
 \n\
 Name: absl_${_NAME}\n\
 Description: Abseil ${_NAME} library\n\