Fix/revisit git-describe enhancement (#11124)

* Fix/keep massaging git-describe result

Ref: https://github.com/neovim/neovim/pull/11117#issuecomment-536416223

* build: revisit generation of version from Git

Fixes "make clean && make", where "auto/versiondef.h" would be missing
since b18b84d - because BYPRODUCTS are apparently removed when cleaning.

This includes the following improvements/changes:

- do not run git-describe during CMake's configure phase just for
  reporting
- do not print with changed Git version (too noisy, simplifies code)

* Move to src/nvim (included before config) for easier flow

* fallback to describe always, write empty include file

* update_version_stamp.lua: use prefix always

src/nvim/CMakeLists.txt

@@ -217,6 +217,34 @@ function(get_preproc_output varname iname)
   endif()
 endfunction()
 
+# Handle generating version from Git.
+set(use_git_version 0)
+if(NVIM_VERSION_MEDIUM)
+  message(STATUS "NVIM_VERSION_MEDIUM: ${NVIM_VERSION_MEDIUM}")
+elseif(${CMAKE_VERSION} VERSION_LESS "3.2.0")
+  message(STATUS "Skipping version-string generation (requires CMake 3.2.0+)")
+elseif(EXISTS ${PROJECT_SOURCE_DIR}/.git)
+  find_program(GIT_EXECUTABLE git)
+  if(GIT_EXECUTABLE)
+    message(STATUS "Using NVIM_VERSION_MEDIUM from Git")
+    set(use_git_version 1)
+  else()
+    message(STATUS "Skipping version-string generation (cannot find git)")
+  endif()
+endif()
+if(use_git_version)
+  # Create a update_version_stamp target to update the version during build.
+  file(RELATIVE_PATH relbuild "${PROJECT_SOURCE_DIR}" "${CMAKE_BINARY_DIR}")
+  add_custom_target(update_version_stamp ALL
+    COMMAND ${LUA_PRG} scripts/update_version_stamp.lua
+      ${relbuild}/config/auto/versiondef_git.h
+      "v${NVIM_VERSION_MAJOR}.${NVIM_VERSION_MINOR}.${NVIM_VERSION_PATCH}"
+    WORKING_DIRECTORY ${PROJECT_SOURCE_DIR}
+    BYPRODUCTS ${CMAKE_BINARY_DIR}/config/auto/versiondef_git.h)
+else()
+  file(WRITE ${CMAKE_BINARY_DIR}/config/auto/versiondef_git.h "")
+endif()
+
 # NVIM_GENERATED_FOR_HEADERS: generated headers to be included in headers
 # NVIM_GENERATED_FOR_SOURCES: generated headers to be included in sources
 # NVIM_GENERATED_SOURCES: generated source files
@@ -245,12 +273,16 @@ foreach(sfile ${NVIM_SOURCES}
 
   get_preproc_output(PREPROC_OUTPUT ${gf_i})
 
+  set(depends "${HEADER_GENERATOR}" "${sfile}")
+  if(use_git_version AND "${f}" STREQUAL "version.c")
+    # Ensure auto/versiondef_git.h exists after "make clean".
+    list(APPEND depends update_version_stamp)
+  endif()
   add_custom_command(
     OUTPUT "${gf_c_h}" "${gf_h_h}"
     COMMAND ${CMAKE_C_COMPILER} ${sfile} ${PREPROC_OUTPUT} ${gen_cflags} ${C_FLAGS_ARRAY}
     COMMAND "${LUA_PRG}" "${HEADER_GENERATOR}" "${sfile}" "${gf_c_h}" "${gf_h_h}" "${gf_i}"
-    DEPENDS "${HEADER_GENERATOR}" "${sfile}"
-    )
+    DEPENDS ${depends})
   list(APPEND NVIM_GENERATED_FOR_SOURCES "${gf_c_h}")
   list(APPEND NVIM_GENERATED_FOR_HEADERS "${gf_h_h}")
   if(${d} MATCHES "^api$" AND NOT ${f} MATCHES "^api/helpers.c$")