Fix g++ compile errors under OSX

[ianfixes]

System

• OS: OSX
• g++: builtin

Issue / Feature Request Summary

Generally, shore up the differences between OSX's g++ (really clang), g++-4.8 (on Travis), and g++-7 (tried it because it was available).

Reported in #24:

Fix for g++ compile errors under OSX.

/Library/Ruby/Gems/2.3.0/gems/arduino_ci-0.1.7/cpp/arduino/Print.h:57:86: error: unknown type name 'PGM_P'
/Library/Ruby/Gems/2.3.0/gems/arduino_ci-0.1.7/cpp/arduino/WString.h:39:16: error: no member named 'isnan' in namespace 'std'

Possibly useful

• https://gcc.gnu.org/onlinedocs/cpp/Common-Predefined-Macros.html