The `r 4.3.3-1` package incorrectly provides `do`, because the DESCRIPTION
file for bundled `Matrix` package contains lines

```
Version: 1.6-5
VersionNote: do also bump src/version.h, inst/include/Matrix/version.h
```

and the `grep Version` in `PKGBUILD` matches both of these, picking the
word `do` from the second line.

This patch fixes the issue by adding a colon to the end of the regexp,
making it similar to the `Package:` regexp that already had a colon.

Other minor improvements in this patch:

- Prefix both regexps with `^` to make them match only at the start of a
  line, making them more robust as these strings could appear inside other
  fields in the file.
- Do lowercase conversion of the package name using the `${parameter,,}`
  bash syntax instead of using `tr`. This makes the code simpler and more
  readable.
- When replacing dashes with dots in the versions, use two slash characters
  instead of one in the bash pattern substitution (`${_ver//-/.}`). This
  replaces all dashes instead of just the first one, making it more robust
  as technically the version could contain more dashes.

Fixes #1

Add -ffat-lto-objects to CFLAGS. Not strictly needed for r itself, but it is inherited by third-party packages (FS#74609)