Handle additional linker arguments correctly

[dmarth]

The environment variable LINK_ARGS allows the user to specify additional linker flags when building the native code. If this environment variable is not set, an empty string is passed on as default value. When calling the linker via the subprocess module, this empty string is interpreted as a file name, which causes the following output.

gcc -pthread -shared build/temp.linux-x86_64-3.6/pyhdf/hdfext_wrap.o -L/usr/local/lib -lmfhdf -ldf -ljpeg -lz -lpython3.6m -o build/lib.linux-x86_64-3.6/pyhdf/_hdfext.cpython-36m-x86_64-linux-gnu.so
gcc: error: : No such file or directory
error: Command "gcc -pthread -shared build/temp.linux-x86_64-3.6/pyhdf/hdfext_wrap.o -L/usr/local/lib -lmfhdf -ldf -ljpeg -lz -lpython3.6m -o build/lib.linux-x86_64-3.6/pyhdf/_hdfext.cpython-36m-x86_64-linux-gnu.so " failed with exit status 1

Instead of an empty string, None should be used as default value. If LINK_ARGS is actually set, its value needs to be splitted to allow multiple arguments.

[fhs]

Thanks!

[eric-wieser]

This was caused by numpy/numpy#12659, which was mostly deliberate but also unexpected.

Note that this patch will break if both numpy < 1.16.0 and LINK_ARGS="-Wl,-rpath=\"path with spaces\"". That's already a corner case though, so probably doesn't matter.

[fphammerle]

Please consider releasing this fix (v0.9.2?)

[eric-wieser]

Note also that this is a behavior change on windows, where backslashes in paths will need extra escaping (shlex.split uses posix escaping)

[fhs]

@fphammerle v0.9.2 has been tagged and uploaded to PyPI.

[fphammerle]

Awesome, thank you!