Added search of INSTALL_DIR, passed in from makefile.  Directories now can be specified without the trailing slash, and the path name is built up correctly (I hope) on Unix, Amiga, and MS-DOS (so special handling for passing strings with a trailing backslash is gone).
Added search of INSTALL_DIR, passed in from makefile.  Directories
now can be specified without the trailing slash, and the path name
is built up correctly (I hope) on Unix, Amiga, and MS-DOS (so special
handling for passing strings with a trailing backslash is gone).

svn path=/trunk/; revision=462

FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot,FreeScienceCommunity/PLPlot