POSIX fixes for standard C compatibility

[zeehio]

Thanks for your handy test header. It is both lightweight and useful!

This pull request contains two commits:

• One commit replaces "\e" by its octal equivalent "\033". "\e" is a GNU extension and does not belong to the ISO C standard. Ref
• The other commit defines (if it is not defined) _POSIX_C_SOURCE as required by man fileno and moves the inclusion of some standard headers after the definition of _POSIX_C_SOURCE as required by man feature_test_macros. cutest.h has been assuming that if unix or APPLE is defined, then POSIX headers are available. This is false if for instance the -ansi flag is passed to the gcc compiler or if the --std=c99 flag is used. By defining _POSIX_C_SOURCE we make sure the POSIX headers are available.

[mity]

Thanks for making it even better :-)