Starting from PHP 5.3.0 syntax:
$current = isset($gd_info['PNG Support']);
will always return TRUE and in the result will incorrectly detects cases:
["PNG Support"]=> bool(false)
["JPEG Support"]=> bool(false)
["FreeType Support"]=> bool(false)
i.e. zabbix frontend will show Yes instead of No.

See attache patch.

With NEW (my suggestion) syntax:
$current = $gd_info['PNG Support'];
those cases detected correctly.

Added:
new checks for:
"JPEG Support" - required for reading background jpeg images for maps,
"FreeType Support" - required for displaying non-ASCII UTF-8 text on the graph correctly and smoothly.

For "JPEG Support" check there is a trick for PHP prior 5.3.0
see http://www.php.net/manual/en/function.gd-info.php

Fixed:
Incorrect detection of 'PNG Support' in the PHP >= 5.3.0

A mistake: "-enable-gettext" -> "-with-gettext".

Fixed a error "constant(): Couldn't find constant LIBXML_DOTTED_VERSION [include/classes/class.frontendsetup.php:417]"
if libxml support missing.

The patch successfully tested with PHP 5.2.17, 5.3.0, 5.3,13, 5.4.0, 5.4.3
The patch based on 2.0.0rc5 rev 27531

As a proof that it's worth to add these checks I'm attaching couple of screenshots.