Release v2.13.0.1

Replaced /usr/local with the use of ${PREFIX} in calc Makefiles.

The ${PREFIX} is not the same as ${T}.  The ${T} specifies
a top level directory under which calc installs things.
While usally ${T} is empty, it can be specific path
as if calc where "chrooted" during an install.
The ${PREFIX} value, during install, is a path between
the top level ${T} install directory and the object
such as an include file.

Corrected a few more typos in Makefile comments.

Added Makefile.local, a file with a single comment.  The main
Makefile includes Makefile.local just before the first all rule.
One may overtide any Makefile setting by modifying Makefile.local.
For example, Makefile.local could force BLD_TYPE:

    HAVE_STRING_H:= YES
    HAVE_TIMES_H:= YES
    SED:= /usr/local/bin/nsed

Added ${LOC_MKF} to specify the make of the file that is
included just before the all file.  So one could place
the above override lines into a different file and call
make changing the ${LOC_MKF} value.  For example:

    make LOC_MKF=Makefile.private clobber all chk

Updated HOWTO.INSTALL to mention Makefile.local.

CHANGES

@@ -28,6 +28,8 @@ The following are the changes from calc version 2.13.0.1 to date:
 
 	make LOC_MKF=Makefile.private clobber all chk
 
+    Updated HOWTO.INSTALL to mention Makefile.local.
+
 
 The following are the changes from calc version 2.13.0.0 to date:
 

HOWTO.INSTALL

@@ -34,7 +34,7 @@ Installing calc from the bzip2-ed tarball in 4 easy steps:
 	The top level Makefile and the custom/Makefile require a modern
 	Make (such as gmake) or an equivalently advanced make.	On many
 	targets, the default make is sufficient.  On FreeBSD for example,
-	one must use gmake instead of make.
+	probably want to use gmake instead of make.
 
 	Some report that under macOS, one has to declare the target
 	to be Darwin.  So for macOS, one might have to do: