when I build LWJGL3 for myself from the Github repository, I do this and it so far always worked.
I too have Windows 7 x64 and Visual Studio 2013 (Community Edition):
- make a clean git clone of https://github.com/LWJGL/lwjgl3.git
- go to "Start" -> "All programs" -> "Visual Studio 2013" -> "Visual Studio Tools"
- the last step opens the Windows Explorer under "C:\Program Files (x86)\Microsoft Visual Studio 12.0\Common7\Tools\Shortcuts"
- there you find the "VS2013 x64 Native Tools Command Prompt" which is a link that executes this: %comspec% /k ""C:\Program Files (x86)\Microsoft Visual Studio 12.0\VC\vcvarsall.bat"" amd64
(I usually create a copy of this link on the "favorites" space in "Start" to save navigating to that VS2013 entry all the time)
This opens the cmd with all variables setup for a native x64 build.
Then I "cd" into lwjgl3, add ant to PATH, and execute ant.