fix building issue on macOS

[cadl]

Hi. I can't build libmc on my macOS, here is the environment and error log:

OS: macOS Catalina 10.15.1
python: conda python 2.7
log:

creating build/temp.macosx-10.6-x86_64-2.7/libmc
gcc -fno-strict-aliasing -I/Users/cadl/.miniconda2/include -arch x86_64 -DNDEBUG -g -fwrapv -O3 -Wall -Wstrict-prototypes -Iinclude -I/Users/cadl/.miniconda2/include/python2.7 -c src/HashkitKetama.cpp -o build/temp.macosx-10.6-x86_64-2.7/src/HashkitKetama.o -fno-strict-aliasing -fno-exceptions -fno-rtti -Wall -DMC_USE_SMALL_VECTOR -O3 -DNDEBUG
warning: include path for stdlibc++ headers not found; pass '-stdlib=libc++' on the command line to use the libc++ standard library instead [-Wstdlibcxx-not-found]
In file included from src/HashkitKetama.cpp:1:
include/hashkit/ketama.h:3:10: fatal error: 'cassert' file not found
#include
^~~~~~~~~
1 warning and 1 error generated.
error: command 'gcc' failed with exit status 1

It's seem like a common issue. ref: pandas-dev/pandas#23424

the reason is:

The compiler standard library defaults to either libstdc++ or libc++, depending on the targetted macOS version - libstdc++ for 10.8 and below, and libc++ for 10.9 and above. This is determined by the environment variable MACOSX_DEPLOYMENT_TARGET or the compiler option-mmacosx-version-min, defaulting to the system version otherwise.

When distuils builds extensions on macOS, it setsMACOSX_DEPLOYMENT_TARGET to the version that python was compiled with, even if the host system / Xcode is newer.

Recent macOS versions of python have a 64-bit only variant built for 10.9 (python.org), and a universal 64/32-bit variant built for 10.6 (python.org) or 10.7 (conda). I am running the conda universal variant, so distutils targets macOS 10.7, despite my system being 10.14, with Xcode 10 which doesn't install libstdc++. Hence the failure I saw.

I copy the changes from pandas pandas-dev/pandas#24274 to solve the issue.

[mckelvin]

This is not good.

In [2]: '10.10' > '10.9'
Out[2]: False

[cadl]

@mckelvin

the type of current_system and python_target is LooseVersion

In [1]: from distutils.version import LooseVersion

In [2]: LooseVersion('10.10') > '10.9'
Out[2]: True

maybe if LooseVersion(python_target) < '10.9' and LooseVersion(current_system) >= '10.9': is a better one

[mckelvin]

Can we compare using the same type? Like python_target < LooseVersion('10.9')?