Install C source code for for debugging help

[Paul Eggert]

In Bug#37527 I proposed a patch to let builders install a compressed 
copy of the C source code for Emacs, so that when users type things like 
'C-h f car RET' they get a button that takes them to the source code for 
'car' instead of giving them a mysterious prompt like "Emacs C source 
dir: ~/" (which is what I get on Debian).

This patch lets builders choose whether to install the compressed 
source. The default is to install it, as the filesystem overhead of 
installing the source code (about 2% of the typical install size) is 
typically not worth worrying about; plus, there is value in encouraging 
builders to better follow the free-software principle of making it easy 
for users to study how Emacs works. Builders can also choose to not 
install the source, or to have Emacs refer to place where the source was 
located when Emacs was built (this last approach does not work in 
typical GNU/Linux distributions, as the build location is not available 
when Emacs is run).

If you run an uninstalled Emacs, it continues to refer to its C source 
directory as before.

I'm mentioning this proposed patch on emacs-devel to see whether anyone 
sees a problem with the idea before I install the patch.