Fixes for the source tarball generation (sage -sdist)

[vbraun]

This branch splits version tagging from source tarball generation. Also, various bugs in sage -sdist are fixed.

Component: build

Author: Volker Braun

Branch/Commit: u/vbraun/sdist_fixes @ d480ee4

Reviewer: R. Andrew Ohana

Issue created by migration from https://trac.sagemath.org/ticket/15472

[vbraun]

Branch: u/vbraun/sdist_fixes

[vbraun]

Commit: d480ee4

[vbraun]

Description changed:

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+++ 
@@ -1 +1 @@
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+This branch splits version tagging from source tarball generation. Also, various bugs in `sage -sdist` are fixed.

[vbraun]

Author: Volker Braun

[vbraun]

New commits:

[d480ee4](https://github.com/sagemath/sagetrac-mirror/commit/d480ee4)	prune the sdist repo
[47faf9e](https://github.com/sagemath/sagetrac-mirror/commit/47faf9e)	fix including gcc tarball in sdist
[9aa6170](https://github.com/sagemath/sagetrac-mirror/commit/9aa6170)	updated sdist documentation
[1b2a228](https://github.com/sagemath/sagetrac-mirror/commit/1b2a228)	source version information in sage -sdist
[2e49430](https://github.com/sagemath/sagetrac-mirror/commit/2e49430)	move updating of version number to separate script
[5a38cf3](https://github.com/sagemath/sagetrac-mirror/commit/5a38cf3)	do not commit during sage -sdist

[ohanar]

Reviewer: R. Andrew Ohana