Makefile rewritten to make custom builds easier

[rhashimoto]

Default WASM builds are checked into the repo in the dist/ directory. If that is all you need you can stop reading now.

For those who want a custom build, the Makefile has been rewritten to make custom builds a little easier. The Makefile will use some variables passed as arguments to make to alter the build configuration on the command line.

The variables are (all empty by default):

• CFILES_EXTRA - Add any C files you want added to the build.
• CFLAGS_EXTRA - Add any C compiler options.
• EMFLAGS_EXTRA - Add any Emscripten options.
• WASQLITE_EXTRA_DEFINES - Add any preprocessor defines.

For example, if you want to build with full text search, use:

make WASQLITE_EXTRA_DEFINES="-DSQLITE_ENABLE_FTS5"

There is also a helper project, sqwab, that can produce a customized wa-sqlite (or official SQLite WASM) build without a local build environment (using GitHub Actions). Instructions are on that project main page.

These changes should make it fairly straightforward to build your own customized WASM, so I won't be attaching the "kitchen sink" build on the release page any more.