Define _XOPEN_SOURCE=501 for xgterm/main.c

`main.c` uses the function `ptsname()` which is declared in `stdlib.h`
as `char *` only if this is defined. Otherwise, the return value
defaults to `int`, which will not correctly convert to a pointer on 64
bit archs.

xgterm/Imakefile

@@ -45,7 +45,8 @@ X11IRAFDIR = ../
 
    MAIN_DEFINES = -DUTMP $(TTYGROUPDEF) $(PUCCPTYDDEF) \
 		  -DOSMAJORVERSION=$(OSMAJORVERSION) \
-		  -DOSMINORVERSION=$(OSMINORVERSION)
+		  -DOSMINORVERSION=$(OSMINORVERSION) \
+		  -D_XOPEN_SOURCE=501
    MISC_DEFINES = /* -DALLOWLOGFILEEXEC */
   EXTRA_DEFINES = -DALLOWLOGGING
   LOCAL_LDFLAGS = -L../lib