Enable POSIX to make heatshrink compile with std=c99 using getopt().

The current target 'make heatshrink' results in "warning: implicit declaration of function ‘getopt’" and undeclared variable errors.
The -std=c99 prevents unistd.h from including getopt.h which makes the compile fail at least on linux.

To make it compile, _POSIX_C_SOURCE=2 is enabled.  Removed including of getopt.h in heatshrink.c again.