build: 'GUIX_CHECK_GUILE_JSON' really checks for Guile-JSON 3.x.
Until now the 'guile' process would always exit with 0, as long
as Guile-JSON is installed, whether it's version 1 or version 3.
* m4/guix.m4 (GUIX_CHECK_GUILE_JSON): Fix array syntax and remove
catch-all 'match' clause.