CMake: Generate helptags during install step.

Fix #1056.

CMakeLists.txt

@@ -177,7 +177,8 @@ if(BUSTED_PRG)
     DEPENDS nvim-test unittest-headers)
 endif()
 
-install(DIRECTORY runtime DESTINATION share/nvim/)
+install(DIRECTORY runtime DESTINATION share/nvim)
+install(SCRIPT ${CMAKE_MODULE_PATH}/GenerateHelptags.cmake)
 
 # Unfortunately, the below does not work under Ninja.  Ninja doesn't use a
 # pseudo-tty when launching processes, because it can put many jobs in parallel

cmake/GenerateHelptags.cmake

@@ -0,0 +1,15 @@
+message(STATUS "Generating helptags.")
+
+execute_process(
+  COMMAND "${CMAKE_CURRENT_BINARY_DIR}/bin/nvim"
+    -u NONE
+    -esX
+    -c "helptags ++t ."
+    -c quit
+  WORKING_DIRECTORY "${CMAKE_INSTALL_PREFIX}/share/nvim/runtime/doc"
+  ERROR_VARIABLE err
+  RESULT_VARIABLE res)
+
+if(NOT res EQUAL 0)
+  message(FATAL_ERROR "Generating helptags failed: ${err}")
+endif()