make --enable-strict configure option be default

This option is off by default for a reason. The strict option has some significant problems between compiler versions. Some harmless warnings become errors with this option, like the trailing semicolon after a class definition. Some warnings appear and disappear depending on the version of gcc.

On some platforms, like Cygwin, this option can spew thousands of worthless warnings depending on the brokenness of the gcc version on that platform at the time.

Some system headers, like windows.h and others, can't properly handle the increased strictness.

For developers, this is an excellent option to use. For a formal release of ICU, it's counterproductive for users compiling ICU. It should be strict for developers and lenient for the users.