Unix build patch: make "./configure; make install" work

The invocation "./configure; make install" wasn't working because
the install target doesn't create the "standard directories", in
particular obj/. This patch simply adds the STDDIRS dependency to
the install-exec target, parallel to the existing std target, which
is the default chosen from a simple "make" invocation.

I'm still concerned that the STDDIRS target is specified in parallel
with the build, because I think the possibility exists that a "make
-j2" parallel can cause things to be written to obj/ before the
subdirectory is created, but I do believe this patch is harmless and
an improvement.

Thanks to Stig Hackvan for bringing it to my attention.

Index: src/unixinst.mak
===================================================================
RCS file: /cvs/ghostscript/gs/src/unixinst.mak,v
retrieving revision 1.31
diff -C2 -r1.31 unixinst.mak
*** src/unixinst.mak	6 May 2003 20:50:10 -0000	1.31
--- src/unixinst.mak	16 Aug 2003 23:34:34 -0000
***************
*** 28,32 ****
  # rules, just in case bindir or scriptdir is a subdirectory of any of these.
  
! install-exec: $(GS_XE)
  	-mkdir -p $(datadir)
  	-mkdir -p $(gsdir)
--- 28,32 ----
  # rules, just in case bindir or scriptdir is a subdirectory of any of these.
  
! install-exec: STDDIRS $(GS_XE)
  	-mkdir -p $(datadir)
  	-mkdir -p $(gsdir)