Fix examples for overriding INSTALL to not suggest hardcoding

'install' since it breaks buildworld after the introduction and
use of 'install -l' in r245752. Overriding INSTALL causes
/usr/bin/install to be used instead of the proper
/usr/src/tools/install.sh which handles the new flag.

Approved by:	bapt
MFC after:	2 weeks

share/examples/etc/make.conf

@@ -97,7 +97,7 @@
 #COPTFLAGS= -O -pipe
 #
 # Compare before install.
-#INSTALL=install -C
+#INSTALL+= -C
 #
 # Mtree will follow symlinks.
 #MTREE_FOLLOWS_SYMLINKS= -L

share/man/man5/make.conf.5

@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd January 21, 2013
+.Dd April 26, 2013
 .Dt MAKE.CONF 5
 .Os
 .Sh NAME
@@ -205,7 +205,7 @@ instead of
 the default install command.
 To install only files for which the target differs or does not exist, use
 .Bd -literal -offset indent
-INSTALL="install -C"
+INSTALL+= -C
 .Ed
 Note that some makefiles (including those in
 .Pa /usr/share/mk )