added git helpers to extract version information from git tags

cmake/git_utils.cmake

+# small cmake module to help extracting the tag and head hash of the current source code tree
+#
+# find_git_root    ... tries to find the .git folder of this repo or any of it's parents
+# git_ready        ... helper function that checks if the current directory is under version control by git
+# git_describe     ... interface to git describe command line call, note only stdout is captured in the output variable
+#
+
+find_package(Git QUIET)
+
+function(find_git_root _git_root)
+
+  set(${_git_root} "GIT_ROOT_DIR-NOTFOUND")
+
+  if(GIT_NOTFOUND)
+    message(INFO "git not found on this system")
+    return()
+  endif()
+
+  set(GIT_PARENT_DIR "${CMAKE_CURRENT_SOURCE_DIR}")
+  set(GIT_PROJECT_ROOT "${GIT_PARENT_DIR}/.git")
+
+  while(NOT EXISTS "${GIT_PROJECT_ROOT}")	# .git dir not found, search parent directories
+    set(GIT_PREVIOUS_PARENT "${GIT_PARENT_DIR}")
+    get_filename_component(GIT_PARENT_DIR ${GIT_PARENT_DIR} PATH)
+    if(GIT_PARENT_DIR STREQUAL GIT_PREVIOUS_PARENT)
+      # We have reached the root directory, we are not in git
+      set(${_git_root} "GIT_ROOT_DIR-NOTFOUND" PARENT_SCOPE)
+      return()
+    endif()
+    set(${_git_root} "${GIT_PARENT_DIR}/.git" PARENT_SCOPE)
+    set(GIT_PROJECT_ROOT "${GIT_PARENT_DIR}/.git")
+  endwhile()
+
+endfunction()
+
+function(git_ready _var)
+
+  find_git_root(GIT_ROOT_DIR)
+
+  if(IS_DIRECTORY ${GIT_ROOT_DIR})
+    set(${_var} "True" PARENT_SCOPE)
+  else()
+    set(${_var} "False" PARENT_SCOPE)
+  endif()
+
+endfunction()
+
+function(git_describe _var)
+
+  find_git_root(GIT_ROOT_DIR)
+  execute_process(COMMAND
+    "${GIT_EXECUTABLE}"
+    describe
+    ${hash}
+    ${ARGN}
+    WORKING_DIRECTORY
+    "${GIT_ROOT_DIR}"
+    RESULT_VARIABLE
+    res
+    OUTPUT_VARIABLE
+    out
+    ERROR_QUIET
+    OUTPUT_STRIP_TRAILING_WHITESPACE)
+
+  set(${_var} "${out}" PARENT_SCOPE)
+endfunction()