compiler: add cc, cxx, and fortran properties

These properties are implemented in the same way in each compiler
package, at least for external specs. So, push the implementation
to the base class for now.

This needs to be revisited, to make the base class less dependent
on C, C++, and Fortran.

lib/spack/spack/build_systems/compiler.py

@@ -165,6 +165,8 @@ def determine_variants(cls, exes: Sequence[Path], version_str: str) -> Tuple:
     #: Flag to activate OpenMP support
     openmp_flag: str = "-fopenmp"
 
+    required_libs: List[str] = []
+
     def standard_flag(self, *, language: str, standard: str) -> str:
         """Returns the flag used to enforce a given standard for a language"""
         if language not in self.supported_languages:
@@ -288,6 +290,39 @@ def archspec_name(self) -> str:
         """Name that archspec uses to refer to this compiler"""
         return self.spec.name
 
+    @property
+    def cc(self) -> Optional[str]:
+        assert self.spec.concrete, "cannot retrieve C compiler, spec is not concrete"
+        if self.spec.external:
+            return self.spec.extra_attributes["compilers"].get("c", None)
+        return self._cc_path()
+
+    def _cc_path(self) -> Optional[str]:
+        """Returns the path to the C compiler, if the package was installed by Spack"""
+        return None
+
+    @property
+    def cxx(self) -> Optional[str]:
+        assert self.spec.concrete, "cannot retrieve C++ compiler, spec is not concrete"
+        if self.spec.external:
+            return self.spec.extra_attributes["compilers"].get("cxx", None)
+        return self._cxx_path()
+
+    def _cxx_path(self) -> Optional[str]:
+        """Returns the path to the C++ compiler, if the package was installed by Spack"""
+        return None
+
+    @property
+    def fortran(self):
+        assert self.spec.concrete, "cannot retrieve Fortran compiler, spec is not concrete"
+        if self.spec.external:
+            return self.spec.extra_attributes["compilers"].get("fortran", None)
+        return self._fortran_path()
+
+    def _fortran_path(self) -> Optional[str]:
+        """Returns the path to the Fortran compiler, if the package was installed by Spack"""
+        return None
+
 
 def _implicit_rpaths(pkg: spack.package_base.PackageBase) -> List[str]:
     detector = spack.compilers.libraries.CompilerPropertyDetector(pkg.spec)