build-sys: add --disable-kill build option

The kill from procps-ng is not always wanted. For example RedHat
seems to prefer kill from util-linux package.

Reported-by: Jaromir Capik <jcapik@redhat.com>
Signed-off-by: Sami Kerola <kerolasa@iki.fi>

Makefile.am

@@ -9,9 +9,6 @@ AM_LDFLAGS = ./proc/libproc-ng.la
 sbin_PROGRAMS = \
 	sysctl
 
-bin_PROGRAMS = \
-	kill
-
 usrbin_exec_PROGRAMS = \
 	free \
 	pgrep \
@@ -27,7 +24,6 @@ usrbin_exec_PROGRAMS = \
 
 dist_man_MANS = \
 	free.1 \
-	kill.1 \
 	pgrep.1 \
 	pkill.1 \
 	pmap.1 \
@@ -41,6 +37,11 @@ dist_man_MANS = \
 	vmstat.8 \
 	w.1
 
+if BUILD_KILL
+bin_PROGRAMS = kill
+dist_man_MANS += kill.1
+endif
+
 if HAVE_NCURSES
 usrbin_exec_PROGRAMS += \
 	slabtop \