This commit log Scilab version at ./configure to ease maintenance. It
uses Scilab include next to the bin directory which will work for both
system-wide install and binaries from scilab.org.
SCILAB_VERSION is define as an automake variable for easier
failure investigation.
Note: the Ubuntu Scilab version is used when available, in the current
CI config there is:
* Scilab 5.5 from scilab.org
* Scilab 6.0 from Ubuntu 18.04
* Scilab 6.1 from Ubuntu 20.04