Embed Git metadata in C/C++ project

This project is loosely based on Andrew Hardin's work. Check the project's README for details. This fork differs in the following ways:

Most of the logic, including the state using a hash of all the variables, is removed. I could not get the project to rebuild when needed, for instance when the input file was modified or a corresponding header file. This implementation simply runs configure_file on each invocation, which doesn't result in a modified output file unless the substitution variables change.
A completely different approach has been taken to the default variables. Personally I have never had a need for commit details like author, e-mail or subject. Instead of using git status, git describe is used to parse the current or latest git tag. The tag is expected to follow semantic versioning and its individual version numbers are extracted into corresponding variables. As a fallback the shortened SHA is used.

Due to the limited regular expression engine in CMake (why do they implement their own?) Perl is used to parse the output of git. Hence perl is needed.

Tag format

An optional "v"
Major version number
Minor version number
Optional patch version number
Optional extra characters other than "-"
Optional deb revision number

Examples:

v1.2.3
v1.2
0.1.0
0.1
v2.4.0+rc1-3
5.0.0alpha-1

In the examples above the last number after the hyphen, if present, is the deb revision. "+rc1" and "alpha" is referred to as "extra" in the following sections.

Variables

git describe provides the following information if HEAD is not the same as the last tag:

Number of commits since last tag
The SHA of the last commit

With --dirty the command also outputs whether working tree is "dirty". All three fields are captured, if present, into variables.

Variables have a default value of either an empty string or the integer -1.

Name	Type	Default	Description
GIT_TAG_VERSION_FULL	string		major.minor(.patch)
GIT_TAG_VERSION_FULL_EXTRA	string		As above but with any additional characters before deb revision, or empty
GIT_TAG_VERSION_MAJOR	int	-1	Major version number from tag
GIT_TAG_VERSION_MINOR	int	-1	Minor version number from tag
GIT_TAG_VERSION_PATCH	int	-1	Patch version number from tag
GIT_TAG_VERSION_EXTRA	string		Any text between sem.ver. and deb revision number
GIT_TAG_VERSION_REVISION	int	-1	deb revision number
GIT_TAG_VERSION_COMMITS	int	-1	Number of commits since last tag
GIT_TAG_VERSION_SHA	string		SHA of last commit or empty if last commit is a tag
GIT_TAG_VERSION_DIRTY	int		1 if working tree is dirty, otherwise 0
GIT_TAG_VERSION_ANY	string		If current commit is a tag, major.min(.patch), otherwise SHA

Usage

Look at the original project for examples. This version is used in a similar way, except it doesn't make sense to use unless tagged version numbers are used.

If the owner of the git project is not the same as the user running this script, the git command will be run with sudo (if installed) or su. This is necessary with recent version of git, which refuses to operate on git projects owned by another user (a result of a CVE patch).

Acknowledgments