Documentation for porting Unix-style build process (configure/make)

Documentation on doing builds of existing libraries with Unix-style configure/make build processes is a bit scarce, and there are some tricks. Would be nice to add a documentation page summarizing porting concerns.

A few things off the top of my head:

• passing CFLAGS, PKG_CONFIG_PATH, and other env variables usually should be done as parameters to configure: emconfigure configure --blah CFLAGS="-s WASM=1" etc
• recommended build steps for multiple libraries / dependencies
  • using out-of-tree build directories for your sanity
  • use --prefix to install multiple libraries into one pseudo system root
  • set include, library paths and PKG_CONFIG_PATH var to point into that root, allowing libraries to find each other
• use emconfigure along with EMCONFIGURE_JS=2 to make sure configure script code is built for the JS target and not for native!
  • (I'm not sure why the default is to build anything as native, it produces incorrect results for all kinds of detections.)
  • running test compilations through emconfigure instead of emmake? For libffi's test suite, this gets them built as node scripts. Not sure this is really ideal though.
• issues with dynamic linking vs static linking?
  • I've sometimes had trouble with -lfoo when the default static library generation created libfoo.a.
  • Shared libraries can produce .so or .dylib files based on the build system, which are actually LLVM bitcode (same as .bc)
  • Not sure how to force the autoconf stack to produce a .bc extension for a static library...?
• recent versions of autoconf support asmjs-unknown-unknown and wasm32-unknown-unknown host triples, so you can pass --host=asmjs or --host=wasm32 on the configure line to flip things into a mode where they know they're cross-compiling! This can help programs know what they're targeting, for instance my in-progress port of libffi needs this or it'll try to build x86_64 assembly stuff.
• best way to obtain proper dependencies? installing autoconf/automake/pkg-config/libtool/etc on Linux, on Mac, and on Windows?
• how to get anything working on Windows??
  • note you can't just exec() a configure script, you need to run it through a shell, and other mysteries