dialog: fix macro redefinition

dialog.h defines MIN and MAX (making sure to undefine the previous
macros if it already exists), but sys/param.h also defines those
macros (without guards) and is included after dialog.h resulting
in both gcc and clang complaining about macro redefiniton

While clang do accept -Wno-macro-redefined to ignore the redefinition
warning, gcc does not [1]

Undefine both macros prior inclusion of sys/param.h to avoid the warning

Reported by:	arichardson

contrib/dialog/util.c

@@ -39,6 +39,8 @@
 #endif
 
 #ifdef HAVE_SYS_PARAM_H
+#undef MIN
+#undef MAX
 #include <sys/param.h>
 #endif
 

gnu/lib/libdialog/Makefile

@@ -15,7 +15,7 @@ MAN=		dialog.3
 
 LIBADD=		ncursesw m
 
-CFLAGS+= 	-I${.CURDIR} -I${DIALOG} -D_XOPEN_SOURCE_EXTENDED -Wno-macro-redefined
+CFLAGS+= 	-I${.CURDIR} -I${DIALOG} -D_XOPEN_SOURCE_EXTENDED
 .PATH:		${DIALOG}
 WARNS?=		1