Document how to build PHP in dev mode (emcc options)

[adamziel]

The emcc needs the following options to build a non-minified output with symbols:

-O0    # don't optimize
-g2     # how much information to retain, use g3 to produce a DWARF file
 -s ASSERTIONS=3     # Add additional assertions to provide more meaningful error messages

[adamziel]

Caveat: -s ASSERTIONS=3 adds the following not-so-useful code to the .js file:

assert(!ENVIRONMENT_IS_WEB, "web environment detected but not enabled at build time.  Add 'web' to `-sENVIRONMENT` to enable.");

assert(!ENVIRONMENT_IS_NODE, "node environment detected but not enabled at build time.  Add 'node' to `-sENVIRONMENT` to enable.");

assert(!ENVIRONMENT_IS_SHELL, "shell environment detected but not enabled at build time.  Add 'shell' to `-sENVIRONMENT` to enable.");

It's not very useful because WEB and WORKER are the same in our case, even if Emscripten wants to distinguish between them.