cmake: support git info in released tarballs

.gitattributes

@@ -1,2 +1,2 @@
 .git* export-ignore
-/CMakeLists.txt export-subst
+version.cmake export-subst

CMakeLists.txt

@@ -228,17 +228,7 @@ elseif(NOT MSVC)
   set(EXTRA_LIBRARIES ${RT} ${PTHREAD} ${DL})
 endif()
 
-file(MAKE_DIRECTORY "${CMAKE_BINARY_DIR}/version")
-find_package(Git QUIET)
-if(Git_FOUND OR GIT_FOUND)
-  message(STATUS "Found Git: ${GIT_EXECUTABLE}")
-  add_custom_target(version ALL "${CMAKE_COMMAND}" "-D" "GIT=${GIT_EXECUTABLE}" "-D" "TO=${CMAKE_BINARY_DIR}/version/version.h" "-P" "src/version.cmake" WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}")
-else()
-  message(STATUS "WARNING: Git was not found!")
-  set(VERSIONTAG "unknown")
-  configure_file("src/version.h.in" "version/version.h")
-  add_custom_target(version ALL)
-endif()
+include(version.cmake)
 
 add_subdirectory(src)
 add_subdirectory(tests)

version.cmake

@@ -0,0 +1,23 @@
+function (write_static_version_header hash)
+  set(VERSIONTAG "${hash}")
+  configure_file("src/version.h.in" "version/version.h")
+  add_custom_target(version ALL)
+endfunction ()
+
+file(MAKE_DIRECTORY "${CMAKE_BINARY_DIR}/version")
+find_package(Git QUIET)
+if ("$Format:$" STREQUAL "")
+  # We're in a tarball; use hard-coded variables.
+  write_static_version_header("release")
+elseif (GIT_FOUND OR Git_FOUND)
+  message(STATUS "Found Git: ${GIT_EXECUTABLE}")
+  add_custom_target(version ALL
+    COMMAND           "${CMAKE_COMMAND}"
+                      "-D" "GIT=${GIT_EXECUTABLE}"
+                      "-D" "TO=${CMAKE_BINARY_DIR}/version/version.h"
+                      "-P" "src/version.cmake"
+    WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}")
+else()
+  message(STATUS "WARNING: Git was not found!")
+  write_static_version_header("unknown")
+endif ()