Allow passing additional DFLAGS to the DMD build

[wilzbach]

There are many predefined ways to build DMD, but AFAIK one can't easily
pass in its own DFLAGS without modifying the Makefile.
With this change the following is now possible:

make -f posix.mak DFLAGS="-version=FooBar"

Learn more about override: https://www.gnu.org/software/make/manual/make.html#Override-Directive

[wilzbach]

Hmm, environment variables are respected too and I think we don't want a global DFLAGS setting to affect the DMD build, so it looks like it's not as simple as this :/

[dlang-bot]

Thanks for your pull request, @wilzbach!

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Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub fetch digger
dub run digger -- build "master + dmd#8089"

[wilzbach]

So looking at the manual there's a good solution:

Variables in make can come from the environment in which make is run. Every environment variable that make sees when it starts up is transformed into a make variable with the same name and value. However, an explicit assignment in the makefile, or with a command argument, overrides the environment.

https://www.gnu.org/software/make/manual/html_node/Environment.html#Environment

which means that:

DFLAGS=
override DFLAGS += -version=MARS $(PIC)

will ignore environment variables, but allow user-defined command line arguments:

> pmake -j20 DFLAGS="-version=FooBar"
CC="c++" dmd -of../generated/linux/release/64/dmd -m64 -vtls -J../generated/linux/release/64 -J../res -L-lstdc++ -version=FooBar -version=MARS -fPIC -w -de

> DFLAGS="-version=FooBar" pmake -j20
CC="c++" dmd -of../generated/linux/release/64/dmd -m64 -vtls -J../generated/linux/release/64 -J../res -L-lstdc++  -version=MARS -fPIC -w -de