Disable Uninitialized Use Warnings for Broken gcc Versions

Some versions of gcc accept unsupported -W flags and run just fine if
there are no warnings, but die with an unsupported flag error if a
warning is encountered.  gcc 4.3 and gcc 4.4 both exhibit this
behavior for -Wno-maybe-uninitialized.  Therefore, if the flag check
for -Wno-maybe-uninitialized succeeds, only use
-Wno-maybe-uninitialized if we are using gcc version 4.7 or greater.
Use -Wno-uninitialized otherwise.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@172543 91177308-0d34-0410-b5e6-96231b3b80d8