build: enable building from a directory which is not a subdirectory of the source directory

Rationale: building `ASAP::O` from a Docker image on macOS inside a
subdirectory of the source directory can be slow because the files
inside the subdirectory are visible from macOS and macOS does not seem
to handle well when many files changed within a virtual machine. It is
therefore faster to compile from a directory that is not mounted on the
Docker image, which cannot be a subdirectory of the source directory.

CMakeIncludes/prepare_version_tag.cmake

@@ -33,18 +33,22 @@ if (DEFINED ENV{CI_COMMIT_REF_NAME})
     return ()
 endif ()
 
-execute_process(COMMAND git describe --tags --abbrev=0
+# Enable running CMake from a directory which is not a subdirectory of
+# ${CMAKE_SOURCE_DIR}. See https://git-scm.com/docs/git#Documentation/git.txt--Cltpathgt
+# for more information on `git -C <path>`.
+set(GIT_CMD git -C "${CMAKE_SOURCE_DIR}")
+execute_process(COMMAND ${GIT_CMD} describe --tags --abbrev=0
         OUTPUT_VARIABLE ASAPO_TAG
         WORKING_DIRECTORY ..)
 string(STRIP ${ASAPO_TAG} ASAPO_TAG)
 
-execute_process(COMMAND git rev-parse --abbrev-ref HEAD
+execute_process(COMMAND ${GIT_CMD} rev-parse --abbrev-ref HEAD
         OUTPUT_VARIABLE BRANCH
         WORKING_DIRECTORY ..)
 string(STRIP ${BRANCH} BRANCH)
 cleanup(BRANCH)
 
-execute_process(COMMAND git rev-parse --short=10 HEAD
+execute_process(COMMAND ${GIT_CMD} rev-parse --short=10 HEAD
         OUTPUT_VARIABLE ASAPO_VERSION_COMMIT
         WORKING_DIRECTORY ..)
 string(STRIP ${ASAPO_VERSION_COMMIT} ASAPO_VERSION_COMMIT)