Xmon 1.5.6 fails to build from source on RHEL5:

~/xmon.1.5.6$ xmkmf
imake -DUseInstalled -I/usr/share/X11/config

~/xmon.1.5.6$ make
gcc -g      -I/usr/include    -Dlinux -D__amd64__ -D_POSIX_C_SOURCE=199309L                             -D_POSIX_SOURCE -D_XOPEN_SOURCE                    -D_BSD_SOURCE -D_SVID_SOURCE                                 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64                                                       -DFUNCPROTO=15 -DNARROWPROTO       -c -o decode11.o decode11.c
gcc -g      -I/usr/include    -Dlinux -D__amd64__ -D_POSIX_C_SOURCE=199309L                             -D_POSIX_SOURCE -D_XOPEN_SOURCE                    -D_BSD_SOURCE -D_SVID_SOURCE                                 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64                                                       -DFUNCPROTO=15 -DNARROWPROTO       -c -o linkl.o linkl.c
gcc -g      -I/usr/include    -Dlinux -D__amd64__ -D_POSIX_C_SOURCE=199309L                             -D_POSIX_SOURCE -D_XOPEN_SOURCE                    -D_BSD_SOURCE -D_SVID_SOURCE                                 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64                                                       -DFUNCPROTO=15 -DNARROWPROTO       -c -o print11.o print11.c
gcc -g      -I/usr/include    -Dlinux -D__amd64__ -D_POSIX_C_SOURCE=199309L                             -D_POSIX_SOURCE -D_XOPEN_SOURCE                    -D_BSD_SOURCE -D_SVID_SOURCE                                 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64                                                       -DFUNCPROTO=15 -DNARROWPROTO       -c -o server.o server.c
gcc -g      -I/usr/include    -Dlinux -D__amd64__ -D_POSIX_C_SOURCE=199309L                             -D_POSIX_SOURCE -D_XOPEN_SOURCE                    -D_BSD_SOURCE -D_SVID_SOURCE                                 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64                                                       -DFUNCPROTO=15 -DNARROWPROTO       -c -o fd.o fd.c
gcc -g      -I/usr/include    -Dlinux -D__amd64__ -D_POSIX_C_SOURCE=199309L                             -D_POSIX_SOURCE -D_XOPEN_SOURCE                    -D_BSD_SOURCE -D_SVID_SOURCE                                 -D_LARGEFILE_SOURCE -D_FILE_OFFSET_BITS=64                                                       -DFUNCPROTO=15 -DNARROWPROTO       -c -o main.o main.c
main.c:79: error: conflicting types for 'gethostname'
/usr/include/unistd.h:843: error: previous declaration of 'gethostname' was here
main.c: In function 'main':
main.c:132: warning: cast to pointer from integer of different size
main.c: In function 'SetUpConnectionSocket':
main.c:402: warning: cast to pointer from integer of different size
main.c: In function 'ReadStdin':
main.c:614: warning: cast from pointer to integer of different size
main.c: In function 'NewConnection':
main.c:824: warning: cast from pointer to integer of different size
make: *** [main.o] Error 1

The cause of the error is a declaration of gethostname at main.c:79 which uses int rather than size_t for the length field.