Use CMAKE_INSTALL_INCLUDEDIR to install headers

Signed-off-by: Clovis Durand <cd.clovel19@gmail.com>

Author: Clovel

Release/src/CMakeLists.txt

@@ -236,9 +236,9 @@ else()
 endif()
 
 if(CPPREST_INSTALL_HEADERS)
-  install(FILES ${HEADERS_CPPREST} DESTINATION include/cpprest)
-  install(FILES ${HEADERS_PPLX} DESTINATION include/pplx)
-  install(FILES ${HEADERS_DETAILS} DESTINATION include/cpprest/details)
+  install(FILES ${HEADERS_CPPREST} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/cpprest)
+  install(FILES ${HEADERS_PPLX} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/pplx)
+  install(FILES ${HEADERS_DETAILS} DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/cpprest/details)
 endif()
 
 if(CPPREST_INSTALL)