Regression in macOS/Conda builds

[traversaro]

It seems that #694 introduced a regression, that went unnoticed due to #695 and #693 .

The CasADi Python bindings are failing with error:

2021-04-24T02:46:53.2116930Z /usr/local/miniconda/envs/test/bin/x86_64-apple-darwin13.4.0-clang++ -DCASADI_VERSION=31 -DHAVE_MKSTEMPS -DUSE_CXX11 -DWITH_DEEPBIND -DWITH_DEPRECATED_FEATURES -DWITH_DL -D_SCL_SECURE_NO_WARNINGS -D_USE_MATH_DEFINES -D_casadi_EXPORTS -I/Users/runner/work/robotology-superbuild/robotology-superbuild/src/casadi/. -I. -I/Users/runner/work/robotology-superbuild/robotology-superbuild/src/casadi/swig/.. -I/Users/runner/work/robotology-superbuild/robotology-superbuild/src/casadi/swig/python -I/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -Icasadi/core/runtime -I/Users/runner/work/robotology-superbuild/robotology-superbuild/src/casadi/casadi/core/../.. -Icasadi/core/../.. -march=core2 -mtune=haswell -mssse3 -ftree-vectorize -fPIC -fPIE -fstack-protector-strong -O2 -pipe -stdlib=libc++ -fvisibility-inlines-hidden -std=c++14 -fmessage-length=0 -isystem /usr/local/miniconda/envs/test/include -fPIC -O3 -DNDEBUG -isysroot /Library/Developer/CommandLineTools/SDKs/MacOSX10.15.sdk -fPIC -Wno-dynamic-class-memaccess -Wno-self-assign -MD -MT swig/python/CMakeFiles/_casadi.dir/__/casadiPYTHON_wrap.cxx.o -MF swig/python/CMakeFiles/_casadi.dir/__/casadiPYTHON_wrap.cxx.o.d -o swig/python/CMakeFiles/_casadi.dir/__/casadiPYTHON_wrap.cxx.o -c swig/casadiPYTHON_wrap.cxx
2021-04-24T02:46:53.2211340Z In file included from swig/casadiPYTHON_wrap.cxx:191:
2021-04-24T02:46:53.2311740Z In file included from /Library/Frameworks/Python.framework/Versions/2.7/include/python2.7/Python.h:88:
2021-04-24T02:46:53.2413770Z /Library/Frameworks/Python.framework/Versions/2.7/include/python2.7/unicodeobject.h:534:5: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
2021-04-24T02:46:53.2512330Z     register PyObject *obj,     /* Object */
2021-04-24T02:46:53.2613360Z     ^~~~~~~~~
2021-04-24T02:46:53.2716080Z /Library/Frameworks/Python.framework/Versions/2.7/include/python2.7/unicodeobject.h:553:5: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
2021-04-24T02:46:53.2814860Z     register PyObject *obj      /* Object */
2021-04-24T02:46:53.2914990Z     ^~~~~~~~~
2021-04-24T02:46:53.3018430Z /Library/Frameworks/Python.framework/Versions/2.7/include/python2.7/unicodeobject.h:575:5: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
2021-04-24T02:46:53.3117280Z     register const wchar_t *w,  /* wchar_t buffer */
2021-04-24T02:46:53.3217400Z     ^~~~~~~~~
2021-04-24T02:46:53.3320730Z /Library/Frameworks/Python.framework/Versions/2.7/include/python2.7/unicodeobject.h:593:5: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
2021-04-24T02:46:53.3421540Z     register wchar_t *w,        /* wchar_t buffer */
2021-04-24T02:46:53.3502380Z     ^~~~~~~~~
2021-04-24T02:46:53.3603790Z In file included from swig/casadiPYTHON_wrap.cxx:191:
2021-04-24T02:46:53.3705950Z In file included from /Library/Frameworks/Python.framework/Versions/2.7/include/python2.7/Python.h:97:
2021-04-24T02:46:53.3809280Z /Library/Frameworks/Python.framework/Versions/2.7/include/python2.7/stringobject.h:173:5: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
2021-04-24T02:46:53.3911580Z     register PyObject *obj,     /* string or Unicode object */
2021-04-24T02:46:53.4012990Z     ^~~~~~~~~
2021-04-24T02:46:53.4116590Z /Library/Frameworks/Python.framework/Versions/2.7/include/python2.7/stringobject.h:174:5: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
2021-04-24T02:46:53.4218900Z     register char **s,          /* pointer to buffer variable */
2021-04-24T02:46:53.4320330Z     ^~~~~~~~~
2021-04-24T02:46:53.4423400Z /Library/Frameworks/Python.framework/Versions/2.7/include/python2.7/stringobject.h:175:5: warning: 'register' storage class specifier is deprecated and incompatible with C++17 [-Wdeprecated-register]
2021-04-24T02:46:53.4523640Z     register Py_ssize_t *len    /* pointer to length variable or NULL
2021-04-24T02:46:53.4625320Z     ^~~~~~~~~
2021-04-24T02:46:53.4726700Z swig/casadiPYTHON_wrap.cxx:8582:30: error: use of undeclared identifier 'PyMemoryView_FromMemory'
2021-04-24T02:46:53.4828160Z     PyObject* buf = arg[i] ? PyMemoryView_FromMemory(reinterpret_cast<char*>(const_cast<double*>(arg[i])), sizes_arg[i]*sizeof(double), PyBUF_READ) : SWIG_Py_Void();
2021-04-24T02:46:53.4928790Z                              ^
2021-04-24T02:46:53.5031360Z swig/casadiPYTHON_wrap.cxx:8592:30: error: use of undeclared identifier 'PyMemoryView_FromMemory'
2021-04-24T02:46:53.5130940Z     PyObject* buf = res[i] ? PyMemoryView_FromMemory(reinterpret_cast<char*>(const_cast<double*>(res[i])), sizes_res[i]*sizeof(double), PyBUF_WRITE) : SWIG_Py_Void();
2021-04-24T02:46:53.5230960Z                              ^
2021-04-24T02:46:53.5332390Z 7 warnings and 2 errors generated.

It seems that for some reason the CasADi build system is finding the system Python 2.7 instead of the Python installed by Conda.