[Plplot-devel] Use $(VAR) instead of @VAR@ in Makefile.am

[Rafael L. <lab...@ps...>]

In doing my last changes, I noticed that in many Makefiles.am, constructs
like @MATWRAP@ and @MKOCTFILE@ are used.  This is bad practice.  Use
$(MATWRAP) and $(MKOCTFILE) instead, which is the standard, advised style.

The reason for this is simple: variables could be replaced in the invocation
of make, like this: 

    make MKOCTFILE=my-mkoctfile

Using the construct @VARIABLE@ prevents doing that.

This is not a particular problem in the PLplot project, since I guess that
people barely need the variable substitution feature.  However, try to
follow the good usage rule, please. 

I will fix those @VAR@ instances in the various Makefile.am, eventually.

-- 
Rafael