fix build with CMake < 3.7

[alexbruy]

Description

Our minimal required CMake version is 3.0.0 which does not have VERSION_GREATER_EQUAL operator. Followup 5cf3396.

Checklist

Reviewing is a process done by project maintainers, mostly on a volunteer basis. We try to keep the overhead as small as possible and appreciate if you help us to do so by completing the following items. Feel free to ask in a comment if you have troubles with any of them.

• Commit messages are descriptive and explain the rationale for changes
• Commits which fix bugs include fixes #11111 in the commit message next to the description
• Commits which add new features are tagged with [FEATURE] in the commit message
• Commits which change the UI or existing user workflows are tagged with [needs-docs] in the commit message and contain sufficient information in the commit message to be documented
• I have read the QGIS Coding Standards and this PR complies with them
• This PR passes all existing unit tests (test results will be reported by travis-ci after opening this PR)
• New unit tests have been added for core changes
• I have run the scripts/prepare-commit.sh script before each commit

[nyalldawson]

I'm gonna keep breaking your build until you decide to upgrade to a modern distro ;)

But +1 to merge if Travis is happy

[alexbruy]

I'm gonna keep breaking your build until you decide to upgrade to a modern distro ;)

I'm open for suggestions :-)