GitLab

- introduce WM_COMPILE_CONTROL variable to convey control information
  into the build rules.

  The convention (as per spack):
      - '+' to select a feature
      - '~' to deselect a feature

  Eg, to select the gold linker, and disable openmp
  (spaces are not required):

      WM_COMPILE_CONTROL="+gold ~openmp"

CONFIG: accept FOAM_EXTRA_LDFLAGS for AMD, gold, Mingw linkers

CONFIG: generalize PROJECT_LIBS (-ldl used almost universally)

  - replaces floating-point equal comparisons in
    `linearEqn`, `quadraticEqn`, and `cubicEqn`,
  - ensures `quadraticEqn` and `cubicEqn` can return `complex` roots,
  - reorders if-branches in `quadraticEqn` and `cubicEqn` to avoid
    zero-equal comparison,
  - adds Kahan's cancellation-avoiding algorithm into `quadraticEqn` and
    `cubicEqn` for the numerically-sensitive discriminant computation,

  - adds/improves `polynomialEqns` tests:
    * adds Test-linearEqn.C
    * adds Test-quadraticEqn.C
    * improves Test-cubicEqn.C