* avoid linker conflicts if we use nlopt-getopt.c with a libc that has getopt

* use nlopt-getopt.h if we are missing getopt or getopt.h

Closes #353

Fixes #349

Dembo paper was in 1983, not 1982, and I added the DOI URL too so it will be easier for others to find than it was for me.

Vector-valued constraints section listed `opt.add_inequality_mconstraint` twice, when it looks like it meant to list both of `opt.add_inequality_mconstraint` and `opt.add_equality_mconstraint`.

Closes #332

Closes #317

Closes #319

Closes #314

Visibility conditioned to specific cmake minimum version and compatible compiler id

By default, the mexFunctions seems to be hidden in the newer Matlab version, this fix force the compiler to keep the symbols visible.

Updating vintage 2010 data.

* Add usage of _MSVC_LANG for windows C++ standard check

* Use MSC_VER instead or WIN32

Detect wrong algorithm string

Also matlab's c++ library might be older and incompatible with the compiler's one,
so mention in the doc the possible workaround.

Added a test.

rm obsolete #ifdef NLOPT_UTIL_H in stogo code

* xtol_rel: L1 norm for whole-vector stopping criterion

* vector_norm: fix uninitialized ret, move branch outside for loop, make it const-correct

* diff_norm: like vector_norm, but for x-oldx

* xtol_abs: test directly when diff_norm tests for weighted xtol_rel

* add double * x_weights to nlopt and nlopt_stopping structures

 - use the weights when checking relative x stopping criterion
 - allow setting the weights like it's implemented for xtol_abs
 - weights default to 1

* don't forget to free the weights; actually declare the weight-related functions

* provide scale support for vector_norm and diff_norm

* add argument checks to nlopt_get_x_weights

* do not allocate x_weights unless requested by

Unallocated (NULL) x_weights behave as if all weigths are set to 1.

* start documenting the new functions

TODO: add *_x_weights functions to C++, Fortran, Python interface,
document those

* fixes, add equations

* Update NLopt_Reference.md

* param names

* check for w < 0 in set_x_weights

* add x_weights to C++ interface and document them

* add x_weights to F77 interface and document them

* add & document x_weights to SWIG-based interfaces

This includes Guile and Python.

* add and document x_weights to Octave&Matlab

* set_x_weights: ISO C90 compatibility

* set_x_weights: provide informative error on w <= 0

* tweaks

Add algorithm enum string conversion

Similar nlopt_get_* functions in options.c always check for
(opt && (opt->n == 0 || argument)), requiring a valid nlopt * opt
and only allowing a null argument when the optimizer is "empty".

nlopt_get_xtol_abs seems to have been left out (13e0f277).