Stop using libraries and header files from external/

[nyh]

Note: this is a controversial suggestion.

Originally, OSv's makefile assumed that the build machine has a C++ compiler, but any libraries or header files which we needed during compilation we took from an "external/*" submodule - where we put a specific known version of these libraries and headers (we usually took those from some version of Fedora). We did this for two reasons: first, repeatable builds (build result doesn't change when the build machine is upgraded), and second cross compilation (distributions often come with a cross-compiler, but not cross-compiled libraries).

But unfortunately, as OSv lived through several releases of distributions, compilers and libraries, the whole "external/" idea crumbled like a house of cards: Certain versions of gcc (remember that gcc itself was not included in external/) required certain versions of libgcc_s, certain versions of libstdc++ only went with certain versions of libgcc_s and gcc, libraries (such as Boost) pre-compiled with one version of libstdc++ would sometimes not work with other versions of libstdc++. So more and more, we changed the makefile to take libraries and header files from the build machine - not from external/.

However, we didn't finish this conversion, there are still libraries and headers in external/, and we still have places which use them and cause bugs - see for example issues #738, #619, #687, and probably #690.

I think we should do the following:

1. Remove libraries and headers from external/
2. Remove the build_env/ option from Makefile
3. Make sure that usr.manifest.skel takes libraries from the build machine, not external/
4. Fix the Makefile of various modules like cloud-init (see On Fedora 23, cloud-init module does does not run #738) which still take libraries or headers from external.
5. For cross, compilation, write a script which fetches the relevant libraries for your cross-compiler. I.e., if you're running on Fedora 23 x86-64 a cross compiler for aarch64, we need to fetch the libraries from the aarch64 distribution of Fedora 23 (but can use the header files already installed on the host).

[wkozaczuk]

After applying the last related commits the following is left:

1. Remove build_env from main makefile (the aarch64 still uses externals).
2. Change nfs implementation to a module so it stops using external/libnfs (see what this smbfs patch did - https://groups.google.com/d/msg/osv-dev/QToC22-QHiA/ejSOMGe7BwAJ).
3. Get rid of gcc.bin and misc.bin in external/aarch64. How do we do it exactly so it works on both Fedora and Ubuntu at least? Modify setup.py to do that if a certain parameter is passed? Would it use a package manager or download like for example openjdk8-fedora app does using wget?