allow to override dmd

[MartinNowak]

No description provided.

[Hackerpilot]

Looks like the CI failure is a result of the changes to linking and code generation in 2.068.

[Hackerpilot]

Actually this broke between 2.068 and 2.068.1.

[Hackerpilot]

I was able to fix it by specifying the path to std.format, but that seems like a hack:

SRC := $(shell find src -name "*.d") \
    $(shell find libdparse/src -name "*.d") \
    $(shell find libdparse/experimental_allocator/src -name "*.d") \
    /home/brian/.dvm/compilers/dmd-2.068.1/src/phobos/std/format.d

I shouldn't have to specify that Phobos files get compiled, should I?

[MartinNowak]

Sorry for not letting you know, we already fixed the issue.
https://trello.com/c/ARtw7Xwl/81-reg2-068-1-linker-error-with-speculative-instantiation-and-inline
dlang/dmd#5055
Looks like we have to release a 2.068.2 soon.

[MartinNowak]

What about this PR?

[Hackerpilot]

I ended up making a similar changes, but using the DC environment variable because that's what Travis uses.

[Hackerpilot]

See 480e6fe