Require commit ID and tag name via CMake vars if git is not found

On OBS for instance, cmake can be invoked like
cmake -DGIT_COMMIT=xxx -DGIT_TAG_NAME=xxx
to collect the required information.

Fixes #308

CMakeLists.txt

@@ -7,15 +7,22 @@ project(linuxdeployqt)
 
 find_program(GIT git)
 
+if("${GIT}" STREQUAL "GIT-NOTFOUND")
+    message(WARNING "Could not find git, commit and tag info cannot be updated")
+
+    if(NOT GIT_COMMIT)
+        message(FATAL_ERROR "Commit ID not set, please call with -DGIT_COMMIT=...")
+    endif()
+
+    if(NOT GIT_TAG_NAME)
+        message(FATAL_ERROR "Tag name not set, please call with -DGIT_TAG_NAME=...")
+    endif()
+else()
     # make sure Git revision ID and latest tag is not stored in the CMake cache
     # otherwise, one would have to reset the CMake cache on every new commit to make sure the Git commit ID is up to date
     unset(GIT_COMMIT CACHE)
     unset(GIT_LATEST_TAG CACHE)
 
-if("${GIT}" STREQUAL "GIT-NOTFOUND")
-    message(FATAL_ERROR "Could not find git")
-endif()
-
     # read Git revision ID and latest tag number
     execute_process(
         COMMAND "${GIT}" rev-parse --short HEAD
@@ -52,6 +59,7 @@ if(NOT GIT_TAG_NAME_RESULT EQUAL 0)
         message(FATAL_ERROR "Failed to determine git tag name")
     endif()
     mark_as_advanced(GIT_TAG_NAME GIT_TAG_NAME_RESULT)
+endif()
 
 # set version and build number
 set(VERSION 1-alpha)