OS X versioning dance.

OS X versioning dance.

Note the difference between the OS X version (10.X) and the kernel version,
it's the latter that Configure knows as $osvers.  Adding a cross-reference
table for these versions rom the NetBSD project.

For OS X 10.6 or above, do not any more use the MACOSX_DEPLOYMENT_TARGET,
the toolchains should work fine without.  Until now the deployment target
was hardwired to 10.3.  This logic comes from
https://rt.perl.org/Public/Bug/Display.html?id=117433

For OS X releases from 10.3 until 10.5, no change, still using
the MACOSX_DEPLOYMENT_TARGET=10.3 for linking.

For OS X releases before 10.3, no change, still not using
the MACOSX_DEPLOYMENT_TARGET=10.3.

New: always add -mmacosx-version-min to ccflags and ldflags from
the env var $MACOSX_DEPLOYMENT_TARGET, if set.  If the var is not set,
set the min from the OS X version, from sw_vers(1).  Setting the var
should become handy for people building and packaging Perl for earlier
OS X versions.

We assume that the toolchain/SDK installed to system will be able to build
for the requested minimum versions and deployment targets, or if it is not,
it should properly warn or die.

Some related tickets, past and present:

https://rt.perl.org/Public/Bug/Display.html?id=126360
https://rt.perl.org/Public/Bug/Display.html?id=123985
https://rt.perl.org/Public/Bug/Display.html?id=123831
https://rt.perl.org/Public/Bug/Display.html?id=117433