hpx: fix incompatible cxxstd with boost and compiling tests (#13660)

* Add a variant for tests and parallelize builds

- Fix a bug with boost defaulting to cxxstd=98 when cxxstd=11 is the
  minimum for hpx
- Disable tests by default and use a variant to enable them if requested
- Enable parallel builds: each process takes up to 1Gb when tests are
  not enabled.

* Remove tests variant

- The variant doesn't change what gets installed. Testing can be
  activated using `spack install --test=root hpx`

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

@@ -43,7 +43,6 @@ class Hpx(CMakePackage, CudaPackage):
     variant('tools', default=False, description='Build HPX tools')
     variant('examples', default=False, description='Build examples')
 
-    depends_on('boost')
     depends_on('hwloc')
     depends_on('python', type=('build', 'test', 'run'))
     depends_on('pkgconfig', type='build')
@@ -77,11 +76,6 @@ class Hpx(CMakePackage, CudaPackage):
     depends_on('papi', when='instrumentation=papi')
     depends_on('valgrind', when='instrumentation=valgrind')
 
-    # TODO: hpx can build perfectly fine in parallel, except that each
-    # TODO: process might need more than 2GB to compile. This is just the
-    # TODO: most conservative approach to ensure a sane build.
-    parallel = False
-
     def cxx_standard(self):
         value = self.spec.variants['cxxstd'].value
         return '-DHPX_WITH_CXX{0}=ON'.format(value)
@@ -125,6 +119,11 @@ def cmake_args(self):
             'ON' if '+cuda' in spec else 'OFF'
         ))
 
+        # Tests
+        args.append('-DHPX_WITH_TESTS={0}'.format(
+            'ON' if self.run_tests else 'OFF'
+        ))
+
         # Tools
         args.append('-DHPX_WITH_TOOLS={0}'.format(
             'ON' if '+tools' in spec else 'OFF'