[CMake] Made existence of local git repo optional.

This is used to minimize the Docker build context when building
an image with OGS inside from the local source tree.

Supply OGS_VERSION CMake variable.

.dockerignore

+.git
+Tests/Data

scripts/cmake/PreFind.cmake

@@ -16,7 +16,16 @@ execute_process(COMMAND ${GIT_EXECUTABLE} status
     RESULT_VARIABLE IS_GIT_REPO
     OUTPUT_QUIET)
 if(IS_GIT_REPO GREATER 0)
-    message(FATAL_ERROR "Source code at ${PROJECT_SOURCE_DIR} is not a git "
-        "repository! Please use git to obtain the source code! See "
-        "https://www.opengeosys.org/docs/devguide/getting-started/get-the-source-code/")
+    set(IS_GIT_REPO FALSE CACHE INTERNAL "")
+    if(DEFINED OGS_VERSION)
+        message(WARNING "Using user-provided OGS_VERSION: ${OGS_VERSION}!")
+        message(WARNING "Submodule setup is skipped!")
+    else()
+        message(FATAL_ERROR "No git repository found at ${PROJECT_SOURCE_DIR}! "
+            "Please use git to obtain the source code! See "
+            "https://www.opengeosys.org/docs/devguide/getting-started/get-the-source-code/"
+            " OR manually set the OGS_VERSION variable.")
+    endif()
+else()
+set(IS_GIT_REPO TRUE CACHE INTERNAL "")
 endif()

scripts/cmake/ProjectSetup.cmake

@@ -28,6 +28,10 @@ include_directories(${CMAKE_CURRENT_SOURCE_DIR})
 
 set(CMAKE_MACOSX_RPATH 1)
 
+if(NOT IS_GIT_REPO)
+    return()
+endif()
+
 # Get version info from Git, implementation based on
 # https://github.com/tomtom-international/cpp-dependencies
 execute_process(

scripts/cmake/SubmoduleSetup.cmake

+if(NOT IS_GIT_REPO)
+    return()
+endif()
+
 # This file initializes the required submodules
 set(OGS_ADDITIONAL_SUBMODULES_TO_CHECKOUT "" CACHE STRING
     "User given submodules which should be checked out by CMake.")