gdk-pixbuf2: stop malloc errors on older systems

[kencu]

need to binwrap these bins, but to do that, need to set compiler.cxx_standard to 2011 due to legacysupport PG

closes: https://trac.macports.org/ticket/66159

Description

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 10.5.8 9L34 i386
Xcode 3.1.4 9M2809

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL?
• checked your Portfile with port lint --nitpick?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of SOME binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@mascguy for port gdk-pixbuf2.

[kencu]

I think a modification to the way the legacysupport PG decides when to allow wrapping bins might be better than forcing ports like this to have the cxx_standard set to 2011+.