Compilation for macOS

[gquere]

From user @ptro46 (thanks!):

On macOS, PATH_MAX is not defined in limits.h, thus it has to be added to the source code:
#define PATH_MAX (1024)

Also the ncurses dependency is named differently than on GNU/Linux, so the Makefile must be altered to link against lcurses:
LDFLAGS = -lpthread -lcurses

After making these changes, ngp2 seems to compile and run as intended.