Fix CMake 3.0 - 3.6 support regression

Replaces `VERSION_GREATER_EQUAL` with `VERSION_EQUAL OR
VERSION_GREATER`.  `VERSION_GREATER_EQUAL` was only added in CMake 3.7.

Fixes #1584.
Closes #1585.

README.md

@@ -339,6 +339,7 @@ skills.
  - Santi Zupancic
  - Jonas Ådahl
  - Lasse Öörni
+ - Pablo Prietz
  - All the unmentioned and anonymous contributors in the GLFW community, for bug
    reports, patches, feedback, testing and encouragement
 

src/CMakeLists.txt

@@ -100,7 +100,10 @@ set_target_properties(glfw PROPERTIES
                       POSITION_INDEPENDENT_CODE ON
                       FOLDER "GLFW3")
 
-if (${CMAKE_VERSION} VERSION_GREATER_EQUAL "3.1.0")
+if (
+    ${CMAKE_VERSION} VERSION_EQUAL "3.1.0" OR
+    ${CMAKE_VERSION} VERSION_GREATER "3.1.0"
+)
     set_target_properties(glfw PROPERTIES C_STANDARD 99)
 else()
     # Remove this fallback when removing support for CMake version less than 3.1