Installing from Source instructions: mention --with-perl

[hadfieldnz]

In following the "Installing from Source" instructions in 1.5 Getting Started - Installing Git on CentOS 7 I found that git-svn was built only if I used configure's --with-perl option to provide a path to the system Perl interpreter, eg: ./configure --prefix=${HOME}/.local --with-perl=/usr/bin/perl I guess that for this to work I had to have the perl, subversion and subversion-perl packages installed.

As the git-svn extension is widely used, I think the instructions in the book should mention this issue, much as they mention the extra prerequisites and installation targets required for the documentation. I am happy to prepare a commit and pull request if it is agreed that this would be a beneficial change.

[ben]

🤔 We're not really trying to be a book about building the Git source code. The guidance we have is enough to get a git binary on most systems, but I'm not sure how much of the --with-* flags we should be documenting. Maybe just a pointer to the place in the Git docs where these are described?

Either way, I'd love to see a PR.