Fix build on macOS

[Rangi42]

Fixes #16
Closes #17

To recap: macOS (and to a lesser extend BSD) has some out-of-the-box limitations that prevent a simple make command from building this repo for me.

• /bin/bash is version 3.2.57 from 2007, which did not support declare -A.
  • Solution: use /usr/bin/env bash, which detects the newer Bash shell installed by Homebrew or MacPorts.
• gcc and cc are links to clang, which does not support -Wl,--gc-sections.
  • Solution: check the OS with $(shell uname -s) and only add that to CFLAGS if it's not macOS. (If uname does not exist, assume Windows, or at least assume "not macOS with the system compiler".)
  • Better solution: do a test run of the compiler with $(shell $(CC) -Wl,--gc-sections ...), and only add that to CFLAGS if it succeeds. (Source: StackOverflow)
• $(shell) is a GNU make feature, not supported by BSD make.
  • Solution: require GNU make (installed as gmake from Homebrew or MacPorts, also commonly installed as gmake on BSD) and rename Makefile to GNUmakefile to indicate this dependency.
• realpath was not available on macOS until version 13 Ventura, and my hardware can't upgrade past 11.7 Big Sur.
  • Solution: if grealpath (another GNU utility commonly installed from Homebrew or MacPorts) exists, use that instead.

This also fixes a couple of things that shellcheck complained about in merge.sh:

• local file="`realpath "$1"`"
  • SC2155 (warning): Declare and assign separately to avoid masking return values.
  • SC2006 (style): Use $(...) notation instead of legacy backticks `...`.
• [[ ! -n ${files[$file]} ]] || return 0
  • SC2236 (style): Use -z instead of ! -n.