Prevent users from spack installing with conflicting fortran compiler

[CamStan]

Describe the problem you're observing

Since Unify currently conflicts with certain fortran compilers (#300, #304), if a user installs with the FC envar set to a conflicting compiler, Unify will build fine, but will fail at runtime when the user attempts to use anything requiring fortran. When a user installs with Spack, Spack sets/overrides FC for the user based on the compiler selected, which prevents the ability to tell Spack to only conflict with the fortran compiler, and not all associated compilers:

- compiler:
    environment: {}
    extra_rpaths: []
    flags: {}
    modules: []
    operating_system: rhel7
    paths:
      cc: /usr/tce/bin/icc-18.0.2
      cxx: /usr/tce/bin/icpc-18.0.2
      f77: /usr/tce/bin/ifort-18.0.2
      fc: /usr/tce/bin/ifort-18.0.2   <---
    spec: intel@18.0.2
    target: x86_64

We do have this documented so that users who intend to use fortran won't try to do so with anything aside from gfortran. However, if a user wanting fortran misses this note and installs with Spack, we could at least have Spack tell them they cannot do so if we had a +fortran variant:

conflicts('%intel', when='+fortran')

This way users that still want to use intel without fortran can still do so, but users wanting fortran will be notified before run time.

Also, clang doesn't have an associated fortran compiler and currently it is unclear what Spack will do there as other complications have prevented testing this.

Proposed Solution

Create an --enable-fortran configuration option that is off by default and doesn't install any fortran related examples, etc when disabled.