use correct GTK+ version for OpenCV based on dependencies

[Flamefire]

(created using eb --new-pr)

This avoids failed installations if system GTK3 is found

[Flamefire]

Test report by @Flamefire

Overview of tested easyconfigs (in order)

• SUCCESS ant-1.10.5-Java-1.8.eb
• SUCCESS OpenCV-4.0.1-foss-2018b-Python-3.6.6.eb
• SUCCESS OpenCV-3.4.5-foss-2018b-Python-2.7.15.eb

Build succeeded for 3 out of 3 (2 easyconfigs in total)
taurusi6608.taurus.hrsk.tu-dresden.de - Linux RHEL 7.9, x86_64, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz (haswell), Python 2.7.5
See https://gist.github.com/1780fbbfc4b0673cda33ad2404220086 for a full test report.

[Flamefire]

Test report by @Flamefire

Overview of tested easyconfigs (in order)

• SUCCESS OpenCV-4.2.0-fosscuda-2019b-Python-3.7.4.eb

Build succeeded for 1 out of 1 (1 easyconfigs in total)
taurusi6605.taurus.hrsk.tu-dresden.de - Linux RHEL 7.9, x86_64, Intel(R) Xeon(R) CPU E5-2680 v3 @ 2.50GHz (haswell), Python 2.7.5
See https://gist.github.com/42774235db94d2da7c9ac5aef15420a5 for a full test report.

[Flamefire]

OpenCV-3.4.5-foss-2018b-Python-2.7.15.eb and OpenCV-4.0.1-foss-2018b-Python-3.6.6.eb show -DWITH_GTK_2_X=ON

OpenCV-4.2.0-fosscuda-2019b-Python-3.7.4.eb doesn't show the flag as expected

[migueldiascosta]

lgtm

[migueldiascosta]

Going in, thanks @Flamefire!