editors/emacs: Disable NATIVECOMP by default for nox flavor

Emacs now includes a feature called native compilation, which compiles
elisp function definitions into native machine code. Unlike
byte-compiled code, natively-compiled elisp runs directly on the CPU,
providing a speedup of 2.5 to 5 times, depending on the workload. [0]

Native compilation requires GCC. Since this is a large dependency,
disable NATIVECOMP by default for the nox flavor, which is often used
for minimal installations.

[0] https://www.gnu.org/software/emacs/manual/html_node/elisp/Native-Compilation.html

Requested by: manu
Sponsored by: The FreeBSD Foundation