sfcgal-config check should be constrained to accept only file

[giohappy]

As reported in [1] if sfcgal-config is given a folder path and not a file path, configuration goes on but sets wrong variables for SFCGAL linking.

Probably line 27 of configure.ac should be changed to:

if [ -x "$SFCGAL_CONFIG" ] && [ -f "$SFCGAL_CONFIG" ]; then

[giohappy]

And probably add a check on SFCGAL_VERSION value to be a valid result…

[vmo]

Duplicates #2784