Make the default SETCAP value to be /sbin/setcap

<Dan_C> DigitalMan: minor install issue noted today - during a fresh (Linux) install on a test box, the build failed because it was unable to execute the "sudo setcap ..." command.  This was on Slackware 15.0, where the setcap executable is located in /sbin , and that directory is not in the $PATH of a normal user, or even when a normal user uses the 'sudo' command.

Also include SETCAP in the set of documented make variables.

install/GNUmakefile

@@ -12,6 +12,7 @@
 # SYMLINK = Don't copy binaries, rather create symlinks in $(SBBSDIR)/exec
 # SBBSDIR = Directory to do install to
 # REPODIR = Directory where source files are cloned
+# SETCAP = First portion of set file capabilities (setcap) command-line (e.g. /sbin/setcap)
 # NOCAP = Set to defeat sbbs3 bind-capabilities (setcap) build target on Linux
 # bcc = Set to use Borland compiler
 # os = Set to the OS name (Not required)
@@ -81,7 +82,7 @@ MKFLAGS	+=	os=$(os)
 
 ifndef NOCAP
 ifeq ($(os),linux)
-	SETCAP 	:=	setcap
+	SETCAP 	:=	/sbin/setcap
 endif
 endif