install: avoid a portability bug when compiling with non-gcc

install: avoid a portability bug when compiling with non-gcc

* src/install.c (extra_mode): Be careful to return only a 0 or 1
value, since this is a "bool" function, and there are still some
compilers for which this is necessary.  Without this change,
Bernhard Voelker reported that the Forte Developer 7 C 5.4 2002/03/09
compiler would produce a malfunctioning "install" binary.  Details in
http://thread.gmane.org/gmane.comp.gnu.coreutils.bugs/17710/focus=17760