Remove Building.configure() from storing stdout and stderr output to a temp file, since these temp files aren't read from anywhere in emscripten codebase(?). In test suite the temp files are stored to a temp dir, and deleted immediately after the test is run. This prevented test suite from reporting build errors to user when the test fails. Related to #694, #695 and #696. Improve error message reporting at the end of a failed configure run.
Output stdout and stderr from external library builds to console only if EMSCRIPTEN_VERBOSE environment variable is defined, to not spam console with information from external tools by default.
Use EM_BUILD_VERBOSE instead of EMSCRIPTEN_VERBOSE as the name of the env. variable that controls whether external configure/build libraries are invoked with logging output to console.
Merge pull request #1543 from juj/configure_debug_logging

Configure debug logging.