Installing the requirements.txt of the gluu-flask is always a chore because of some or the other bug that crops up with the Swig related compilation of M2Crypto library.
After a clean install of homebrew on El Capitan on the Mac, the version incompatibility issues disappeared, but still there needs to be some workaround to set this up.
# set up the pre requisites
$ brew install python
$ brew install swig
$ brew install openssl
$ pip install virtualenv
# now git clone the repo
$ cd /some/where/gluu-flask
$ virtualenv env
$ source env/bin/activate
$ env LDFLAGS="-L$(brew --prefix openssl)/lib" \
> CFLAGS="-I$(brew --prefix openssl)/include" \
> SWIG_FEATURES="-cpperraswarn -includeall -I$(brew --prefix openssl)/include" \
> pip install -r requirements.txt