Makefile: Use a single version of dmd, from path

[Geod24]

The usage of two versions of DMD seems to be to work around old compiler issues.
Nowadays, DMD 2.079.0 is old enough (released 2018-03-01)
that it can be set as the mainline.
Finally, instead of used fixed locations that are dependent on the developer's setup,
simply use the DMD that should be in the PATH.

[Geod24]

Ping @WalterBright : This is fairly simple.

[Geod24]

Ping @WalterBright

[WalterBright]

I use different compilers all the time To avoid annoying surprises, and to know what is being used, I hard code them in the makefile.

I tried changing the build compiler to 2.092, and sure enough it builds an executable that doesn't work:

Fatal error: more than 32767 symbols in object file

Don't know why at the moment.

[WalterBright]

Something has gone wrong with compiling insidx in cgobj.d

[WalterBright]

Ok, the problem with the 2.092 code generation is fixed. dlang/dmd#12515

But a new problem has cropped up. I use my Windows XP machine to run the DMC test suite, but dmd 2.092 no longer works on Windows XP, the system says it is not a valid EXE file.

https://issues.dlang.org/show_bug.cgi?id=21915

So, changing the makefile to use the latest compiler doesn't work.

[Geod24]

@WalterBright : The next release will contain the fix. Plus, nothing prevents you from using make DMD=e:\dmd2.079. Can we get this in so I can rebase the CI PR ?

[Geod24]

Ping @WalterBright

[WalterBright]

I've gotten everything to work with 2.092, so it's fine.