harmonize build flags between build-from-source and upstream-packaged images

[neomantra]

Upstream packaged images (e.g. centos via RPM) have a set of modules and build flags. See https://github.com/openresty/openresty-packaging/blob/master/rpm/SPECS/openresty.spec#L145

Harmonize these flags and make them customizable with build arguments.

For example:

ARG RESTY_LUABIND_XCFLAGS="-DLUAJIT_NUMMODE=2 -DLUAJIT_ENABLE_LUA52COMPAT"

Other things to look at are cc-opt and ld-opt. Related to #108

[neomantra]

@agentzh At this issue comment, you point out using using a pcre tree.

If I am using --with-pcre (like here, do I need to also use those cc and ld overrides? Or does the OpenResty build system take care of that when --with-pcre is specified?

[neomantra]

As of 1.15.8.1-3, I've implemented:

• same PCRE build options and exports (add pcre build support for alpine & alpine-fat #108)
• the LuaJIT xcflags (although done differently than written above)
• added -DNGX_LUA_ABORT_AT_PANIC

What's left is parity of the included modules....