I have installed the latest development version of Emacs on my Mac from the
source (this is the first time I'm installing Emacs from source), by
cloning the Git repository here:
https://savannah.gnu.org/git/?group=emacs ('Emacs
source repository' under 'Anonymous clone'), and then executing `make` in
the repo's top directory.

It didn't go smoothly, but only because I didn't have some prerequisites
installed (namely, `Autoconf` and `gnutls`, and the OS X command-line
tools). Once I'd installed the prerequisites (`Autoconf` and `gnutls` with
`Homebrew`, and the OS X command-line tools by running `xcode-select
--install`), the `make` script completed successfully.

However, when I open the Emacs GUI (<installation
directory>/nextstep/Emacs), there's no splash screen, and instead of the
usual *GNU Emacs* buffer, the *scratch* buffer opens initially.

More importantly, when I try to inspect variables by executing `C-h v`, the
following error message appears in the echo buffer: 'Cannot open load file:
No such file or directory, help-fns'.

Why doesn't the `C-h v` command work, and how can I view and set a
variable's value as well as inspect the info about a variable?

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I run macOS Sierra version 10.12.5 (the latest release as of today).