Closes #3256

This reverts commit 717e821b. It's
much more convenient to do 'make OPTIMIZE=0'.

This cuts 'make install -j6' on my laptop from 170s to 134s.

Taken from Mesa configure script:

https://github.com/mesa3d/mesa/blob/17.2/configure.ac#L405-L427

Fixes #3113

Revert 82b7f0e8, cd8bc06e, c3db9e6f

This breaks the tarball job: https://hydra.nixos.org/build/95714570

This should finally allow us to address all cases of build errors due to
differences between release tarballs and building from git.

See also https://github.com/NixOS/nix/issues/506#issuecomment-507312587

This is to avoid confusion as in commit
a0d29040.

Also give a helpful error message on what package the user likely
has to install to make it work.

Our use of boost::coroutine2 depends on -lboost_context,
which in turn depends on `-lboost_thread`, which in turn depends
on `-lboost_system`.

I suspect that this builds on nix only because of low-level hacks
like NIX_LDFLAGS.

This commit passes the proper linker flags, thus fixing bootstrap
builds on non-nix distributions like Ubuntu 16.04.

With these changes, I can build Nix on Ubuntu 16.04 using:

    ./bootstrap.sh
    ./configure --prefix=$HOME/editline-prefix \
      --disable-doc-gen \
      CXX=g++-7 \
      --with-boost=$HOME/boost-prefix \
      EDITLINE_CFLAGS=-I$HOME/editline-prefix/include \
      EDITLINE_LIBS=-leditline \
      LDFLAGS=-L$HOME/editline-prefix/lib
    make

where

* g++-7 comes from gcc-7 from
  https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test,
* editline 1.14 from https://github.com/troglobit/editline/releases/tag/1.14.0
	was installed into `$HOME/editline-prefix`
  (because Ubuntu 16.04's `editline` is too old to have the function nix uses),
* boost 1.66 from
	https://www.boost.org/doc/libs/1_66_0/more/getting_started/unix-variants.html
	was installed into $HOME/boost-prefix (because Ubuntu 16.04 only has 1.58)

As is normal for autoconf-based projects.

For example, it is a common use case to do

    ./configure CXXFLAGS=-O0

This did not work for nix until now, because the `CXXFLAGS=` declaration
would unconditionally erase what the user had specified.

The custom `OPTIMIZE` flag is removed, but the default `-O3` is retained;
autoconf would default to `-g -O2` by default otherwise as documented on:

https://www.gnu.org/software/autoconf/manual/autoconf-2.60/html_node/C-Compiler.html
https://www.gnu.org/software/autoconf/manual/autoconf-2.60/html_node/C_002b_002b-Compiler.html

That was incorrect, because checking the dirent type already requires
a working compiler.

It had the effect that setting e.g. `: ${CFLAGS=""}` before `AC_PROG_CC`
as per `AC_PROG_CC`'s documentation would have no effect, because
`AC_STRUCT_DIRENT_D_TYPE` would automatically set CFLASGS.

(In a followup commit `: ${CFLAGS=""}` will be used, so it's important
to get this working first.)

And probably many other distributions.

Until now, ./configure would fail silently printing a warning

    ./configure: line 4621: AX_CXX_COMPILE_STDCXX_17: command not found

and then continuing, later failing with a C++ #error saying that some C++11
feature isn't supported (it didn't even get to the C++17 features).

This is because older distributions don't come with the
`AX_CXX_COMPILE_STDCXX_17` m4 macro.

This commit vendors that macro accordingly.

Now ./configure complains correctly:

    configure: error: *** A compiler with support for C++17 language features is required.

On Ubuntu 16.04, ./configure completes if a newer compiler is used, e.g. with
gcc-7 from https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test
using:

    ./bootstrap.sh
    ./configure CXX=g++-7 --disable-doc-gen --with-boost=$(nix-build --no-link '<nixpkgs>' -A boost.dev)

And probably other Linux distributions with long-term support releases.

Also update manual stating what version is needed;
I checked that 1.14 is the oldest version with which current nix compiles,
and added autoconf feature checks for some functions added in that release
that nix uses.

This turns previous compiler errors complaining about missing files
into proper ./configure time errors telling the user which version
of boost is required.

The unbalanced single-quotes cause many editor syntax highlighters
to interpret the rest of the file as a string literal, making it easier
to make syntax mistakes in absence of proper highlighting.

This fixes warnings about "structured binding declaration" requiring
"-std=c++1z".

This tells Nix to not build the shared libraries.

This reverts commit e8b0efdc.

This reverts commit 80f464d9.

This patch attempts to solve the regression introduced in e8b0efdc
(#2607).

The manual reference was commented out, and the sole reference to this
program other than the configure script. Removed both.

"configure.ac" is used to generate "configure", which is supposed to be
run with /bin/sh (as suggested by the shebang line of "configure"). As a
result it is a good idea to remove any /bin/sh-incompatible syntax from
configure.ac. Otherwise, systems that do not use Bash as their /bin/sh
are unable to run "configure" due to syntax errors.

Fixes #2396.

It's all dead code since 2014 (commit 0c6d62cf).

Happily the failing tests should prevent anyone from using such a Nix
in situations where they expect sandboxing to be on,
which would otherwise be a risk.

This is needed for new arches where libseccomp support doesn't exist
yet.

Fixes #1878.