Makefile.{dmd,ldc}: Pin C standard to C99

[OPNA2608]

Code is definitely not C23-compatible, and I imagine C99 is prolly what this was initially targeting.

https://hydra.nixos.org/build/317828012

gcc -c src/asm/acme.c -o src/asm/acme.o
In file included from src/asm/acme.h:10,
                 from src/asm/acme.c:31:
src/asm/config.h:53:17: error: 'bool' cannot be defined via 'typedef'
   53 | typedef int     bool;
      |                 ^~~~
src/asm/config.h:53:17: note: 'bool' is a keyword with '-std=c23' onwards
src/asm/config.h:53:1: warning: useless type name in empty declaration
   53 | typedef int     bool;
      | ^~~~~~~
src/asm/acme.c: In function 'acme_assemble':
src/asm/acme.c:157:32: error: too many arguments to function 'Output_get_final_data'; expected 0, have 1
  157 |                         return Output_get_final_data(length);
      |                                ^~~~~~~~~~~~~~~~~~~~~ ~~~~~~
In file included from src/asm/acme.c:44:
src/asm/output.h:43:14: note: declared here
   43 | extern char* Output_get_final_data();
      |              ^~~~~~~~~~~~~~~~~~~~~

The other Makefiles prolly have this issue too, but the LDC one is the only one we're really using downstream in Nixpkgs.