Bugfix: Install missing compilers for dependency packages (#11175)

Compilers are treated separately from other dependencies in Spack.
#10761 added the option to automatically install compilers when a
package specifies using a compiler that is not available in Spack.
However, this did not work correctly for dependency packages (it
would only build a compiler for the root of an install DAG). This
commit enables the building of compilers for dependency packages.

lib/spack/spack/package.py

@@ -1441,17 +1441,16 @@ def do_install(self, **kwargs):
             dep_kwargs['explicit'] = False
             dep_kwargs['install_deps'] = False
             for dep in self.spec.traverse(order='post', root=False):
+                if spack.config.get('config:install_missing_compilers', False):
+                    tty.debug('Bootstrapping {0} compiler for {1}'.format(
+                        self.spec.compiler, self.name
+                    ))
+                    comp_kwargs = kwargs.copy()
+                    comp_kwargs['explicit'] = False
+                    comp_kwargs['install_deps'] = True
+                    dep.package.bootstrap_compiler(**comp_kwargs)
                 dep.package.do_install(**dep_kwargs)
 
-        # Then, install the compiler if it is not already installed.
-        if install_deps:
-            tty.debug('Boostrapping {0} compiler for {1}'.format(
-                self.spec.compiler, self.name
-            ))
-            comp_kwargs = kwargs.copy()
-            comp_kwargs['explicit'] = False
-            self.bootstrap_compiler(**comp_kwargs)
-
         # Then, install the package proper
         tty.msg(colorize('@*{Installing} @*g{%s}' % self.name))