[BUGFIX] Check for imageftbbox() existence before usage

Install tool system environment check calls imageftbbox() without
checking if the function exists and may fatal. The patch adds a
function_exists() and introduces an error status message.

Resolves: #49458
Releases: 6.2

Change-Id: Ibb3be59d6506632db18d68e3394fd5a4b048da81
Reviewed-on: https://review.typo3.org/21783
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Alexander Schnitzler
Tested-by: Alexander Schnitzler
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn