compiler flags: add cxx98 standard support (#7601)

The following improvements are made to cxx standard support
(e.g. compiler.cxxNN_flag functions) in compilers:

* Add cxx98_flag property
* Add support for throwing an exception when a flag is not supported (previously
  if a flag was not supported the application was terminated with tty.die)
* The name of the flag associated with e.g. c++14 standard support changes for
  different compiler versions (e.g. c++1y vs c++14). This makes a few corrections
  on what flag to return for which version.
* Added tests to confirm that versions report expected flags for various c++
  standards (or raise an exception for versions that don't provide a given cxx
  standard)

Note that if a given cxx standard is the default, the associated flag property will
return ""; cxx98 is assumed to be the default standard so this is the behavior for
the associated property in the base compiler class.

Package changes:

* Improvements to the boost spec to take advantage of the improved standard
  flag facility.
* Update the clingo spec to catch the new exception rather than look for an
  empty flag to indicate non-support (which is not part of the compiler flag API)

var/spack/repos/builtin/packages/clingo/package.py

@@ -44,7 +44,9 @@ class Clingo(CMakePackage):
     depends_on('python')
 
     def cmake_args(self):
-        if not self.compiler.cxx14_flag:
+        try:
+            self.compiler.cxx14_flag
+        except UnsupportedCompilerFlag:
             InstallError('clingo requires a C++14-compliant C++ compiler')
 
         args = ['-DCLINGO_BUILD_WITH_PYTHON=ON',