Add MSVC `-GL` workaround

[sklam]

On windows, HEAD is suffering from a segfault problem when running the testsuite. The problem is associated with the use of -GL flag on MSVC. The flag enables whole-program optimization. I cannot find any mentioning of this issue on LLVM issue tracker.

The patch simply discard -GL from CFLAGS and CXXFLAGS. These environment variables can be set by user or build system (e.g. conda-build).

an ipython reproducer

!python -m llvmlite.tests -vb
ct = 0
while _exit_code == 0:
    !python -m llvmlite.tests -vb
    ct += 1
print("count", ct)

This loops until it fails because it may takes several trial to reproduce. Usually within 3 runs.

[sklam]

BFID: llvmlite_310