build: use GNUInstallDirs where possible

Some installation destinations were already converted to the
GNUInstallDirs standard in e3cf4d0cf. This patch converts the share and
include directories (typically /usr/share and /usr/include respectively)
as well.

Type: improvement
Change-Id: Id481b16604f3f52203166a5c0647f43d6eb05239
Signed-off-by: Wim de With <wf@dewith.io>
diff --git a/src/cmake/api.cmake b/src/cmake/api.cmake
index 0e273a7..10e89d7 100644
--- a/src/cmake/api.cmake
+++ b/src/cmake/api.cmake
@@ -70,7 +70,7 @@
   )
   install(
     FILES ${output_name}
-    DESTINATION share/vpp/api/${dir}/
+    DESTINATION ${CMAKE_INSTALL_DATADIR}/vpp/api/${dir}/
     COMPONENT ${component}
   )
 endfunction()
@@ -101,7 +101,7 @@
   )
   install(
     FILES ${output_name}
-    DESTINATION include/vapi
+    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/vapi
     COMPONENT vpp-dev
   )
 endfunction ()
@@ -128,7 +128,7 @@
   )
   install(
     FILES ${output_name}
-    DESTINATION include/vapi
+    DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/vapi
     COMPONENT vpp-dev
   )
 endfunction ()