Added the possibility to build the static cosoco library without having the XCSP3 parser

[tomosonalawa]

In this commit, a few things were made:

• a compiler definition USE_XCSP3 is used to know if we are allowed to use the XCSP3 parser headers
• a cmake option 'NO_XCSP3' defined such that by default, XCSP3 is still used
• define the cosoco binary only if XCSP3 is allowed
• define the file 'utils/Constants.h' that provide the definition of 'STAR' if XCSP3 is not allowed, or include the XCSP3Constants.h to retrieve the definition of 'STAR'
• provide a first implementation of the build-lib.sh script, meant to build the static library that has no dependency to libxml2 or XCSP3-Parser

[tomosonalawa]

Added new content:

• some framework for testing (optional)
• first pass at memory cleaning to reduce the amount of memory leaks

[tomosonalawa]

The definition of those path are now set to the build.sh file. That way, we can safely build the different versions in different folders without having to fear that one might contaminate the other.

[audemard]

Thanks for this pull request. I will work on it next week.

Best.

Gilles.