support VERBOSE=1 from makefiles

[marler8997]

This allows makefile users to see verbose build output with make VERBOSE=1.

Note that the posix version of the makefiles don't need any changes because GNU make automatically promotes variables set on the command-line to environment variables. This means they will propagate down to the build.d process. However, Digital Mars Make on windows does not set them as environment variables, so they must be explicitly propagated each time make is recursively called and when we call build.d.

Side Note: windows makefiles are missing some variables that need to be propagated, something that we should fix in another change.

That being said, I wasn't able to edit the root makefiles win32.mak and win64.mak to propagate the VERBOSE variable because autotester kept giving me errors like this:

Error on line 3: expecting target : dependencies

https://auto-tester.puremagic.com/show-run.ghtml?projectid=1&runid=3867801&isPull=true
https://auto-tester.puremagic.com/show-run.ghtml?projectid=1&runid=3867802&isPull=true
https://auto-tester.puremagic.com/show-run.ghtml?projectid=1&runid=3867859&isPull=true
https://auto-tester.puremagic.com/show-run.ghtml?projectid=1&runid=3867868&isPull=true

It looks like the autotester is depending on those files to look a certain way. So on windows, I could only make the VERBOSE flag work if you invoke make on src/win32.mak or src/win64.mak directly.

[dlang-bot]

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