Fix gawk-ism in Makefile

[daviesrob]

The awk script used to extract htscodec's version string (if git describe fails or returns a hash) used a three-argument form of match, which is a GNU extension. Rewrite for better compatibility with other awk implementations, notably mawk, which supplies awk in some Cirrus-CI images.

This doesn't appear to show up on HTSlib's tests, which get a suitable version number from git describe, but does on SAMtools' where HTSlib (and so htscodecs) is cloned separately.

Also fix the cirrus-ci badge, which linked to the wrong location.

[jkbonfield]

The awkism change works, and is simpler.

However in review I've noticed the regexp has always been wrong as it doesn't permit more than one digit. At the risk of falsely accepting 12.34..56 we could just replace the regexp with /[0-9.]+/ which is much easier to understand.

Edit: or /[0-9]+(\.[0-9]+)*/ to return 12.34 instead of 12.34..56, but really I don't care as both are assuming something wrong and the version string is our string in the first place so don't create duff ones and the simplest regexp works fine. (And arguably it could just be arg 2 from AC_INIT and it'd accept string versions, but let's not go there!)

[daviesrob]

Multiple digits should now be accepted.