Remove race condition during `make dist`

Introduced by 85828b8f

This is running 2 git processes in parallel, which, if unlucky can cause
either of them to fail with "File already exists" error.

cmake/make_dist.cmake.in

@@ -43,6 +43,13 @@ IF(GIT_EXECUTABLE)
   MESSAGE(STATUS "Running git checkout-index")
   EXECUTE_PROCESS(
     COMMAND "${GIT_EXECUTABLE}" checkout-index --all --prefix=${PACKAGE_DIR}/
+    WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
+    RESULT_VARIABLE RESULT
+  )
+  IF(NOT RESULT EQUAL 0)
+    SET(GIT_EXECUTABLE)
+  ENDIF()
+  EXECUTE_PROCESS(
     COMMAND "${GIT_EXECUTABLE}" submodule foreach "${GIT_EXECUTABLE} checkout-index --all --prefix=${PACKAGE_DIR}/$path/"
     WORKING_DIRECTORY ${CMAKE_SOURCE_DIR}
     RESULT_VARIABLE RESULT