Use of HSL linear solvers in Uno

[amontoison]

I suggest to split the detection of libraries related to HSL linear solvers.
Right now, only MA57 is interfaced, but if more solvers are added, we want something like this:

if findlibrary(hsl)
  ... # compile all HSL interfaces
else
  if findlibrary(ma57)
    ... # compile the interface for MA57
  endif
  if findlibrary(ma27)
    ... # compile the interface for MA27
  endif
endif

This way, we can also determine if we explicitly linked with libHSL, and thus decide whether or not we can use the routine LIBHSL_isfunctional to ensure at runtime that HSL solvers are available.
This is particularly relevant for Julia users, where Uno is always compiled with a "dummy" version of libHSL.
It allows them to easily switch to the HSL solvers if they can install the official libHSL (like academic users) with HSL_jll.jl.

[cvanaret]

Implemented in commit 20e0b2c.