fix macOS use of -dead_strip_dylibs, install major version lib symlinks.

Under macOS, to reduce dependency chains, we remove functions and
data that are unreachable by the entry point or exported symbols.
In particular, the macOS linker is used with "-dead_strip" by default.

While calc on macOS will execute if linker used with "-dead_strip_dylibs"
and CUSTOM is defined, other applications that use libcalc but not
libcustcalc (such as sample_many and sample_rand) will fail to execute
due to missile symbols.  Therefore "-dead_strip_dylibs" is not used
by default when ALLOW_CUSTOM is "-DCUSTOM" under macOS.
Thanks to GitHub user @TurtleWilly for bringing to our attention,
problems related to use of  "-dead_strip_dylibs".

When installing shared libraries, libcalc.x.y.z will be a link
to the libcalc shared library and libcustcalc.x.y.z will be a link
to the libcustcal shared library.

Makefile.config

@@ -1313,6 +1313,7 @@ VER= 2.14.3
 #
 LIB_EXT= .so
 LIB_EXT_VERSION= ${LIB_EXT}.${VERSION}
+LIB_EXT_VER= ${LIB_EXT}.${VER}
 
 # standard utilities used during make
 #