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/boost/package.py

@@ -126,6 +126,11 @@ class Boost(Package):
         variant(lib, default=(lib not in default_noinstall_libs),
                 description="Compile with {0} library".format(lib))
 
+    variant('cxxstd',
+            default='default',
+            values=('default', '98', '11', '14', '17'),
+            multi=False,
+            description='Use the specified C++ standard when building.')
     variant('debug', default=False,
             description='Switch to the debug version of Boost')
     variant('shared', default=True,
@@ -250,6 +255,26 @@ def determine_bootstrap_options(self, spec, withLibs, options):
             if '+python' in spec:
                 f.write(self.bjam_python_line(spec))
 
+    def cxxstd_to_flag(self, std):
+        flag = ''
+        if self.spec.variants['cxxstd'].value == '98':
+            flag = self.compiler.cxx98_flag
+        elif self.spec.variants['cxxstd'].value == '11':
+            flag = self.compiler.cxx11_flag
+        elif self.spec.variants['cxxstd'].value == '14':
+            flag = self.compiler.cxx14_flag
+        elif self.spec.variants['cxxstd'].value == '17':
+            flag = self.compiler.cxx17_flag
+        elif self.spec.variants['cxxstd'].value == 'default':
+            # Let the compiler do what it usually does.
+            pass
+        else:
+            # The user has selected a (new?) legal value that we've
+            # forgotten to deal with here.
+            tty.die("INTERNAL ERROR: cannot accommodate unexpected variant ",
+                    "cxxstd={0}".format(spec.variants['cxxstd'].value))
+        return flag
+
     def determine_b2_options(self, spec, options):
         if '+debug' in spec:
             options.append('variant=debug')
@@ -299,6 +324,17 @@ def determine_b2_options(self, spec, options):
                 'toolset=%s' % self.determine_toolset(spec)
             ])
 
+        # Other C++ flags.
+        cxxflags = []
+
+        # Deal with C++ standard.
+        if spec.satisfies('@1.66:'):
+            options.append('cxxstd={0}'.format(spec.variants['cxxstd'].value))
+        else:  # Add to cxxflags for older Boost.
+            flag = self.cxxstd_to_flag(spec.variants['cxxstd'].value)
+            if flag:
+                cxxflags.append(flag)
+
         # clang is not officially supported for pre-compiled headers
         # and at least in clang 3.9 still fails to build
         #   http://www.boost.org/build/doc/html/bbv2/reference/precompiled_headers.html
@@ -306,10 +342,13 @@ def determine_b2_options(self, spec, options):
         if spec.satisfies('%clang'):
             options.extend(['pch=off'])
             if '+clanglibcpp' in spec:
+                cxxflags.append('-stdlib=libc++')
                 options.extend(['toolset=clang',
-                                'cxxflags="-stdlib=libc++"',
                                 'linkflags="-stdlib=libc++"'])
 
+        if cxxflags:
+            options.append('cxxflags="{0}"'.format(' '.join(cxxflags)))
+
         return threadingOpts
 
     def add_buildopt_symlinks(self, prefix):

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',