rexm used to assume certain paths existed and then `_putenv()` them into `PATH`, which is very error prone. `make` was also assumed, though `mingw32-make` seems to be more common on Windows. Since this shifts things to expect the user to setup their environment, I added some explanation to the README as well.
OP_TESTLOG still uses a hardcoded path when doing web testing (`BUILD_TESTING_WEB`,) unfortunately.
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