component/default: system gcc can be used only if all gcc, g++ and gfortran...

component/default: system gcc can be used only if all gcc, g++ and gfortran exist and their versions are same.

component/defaults.cfg

@@ -42,13 +42,22 @@ min_version = 5.4
 init =
   import os, subprocess
   parse_version = lambda ver: tuple(map(int, ver.strip().split('.')))
-  try:
-    current = subprocess.check_output(('gcc', '-dumpfullversion'),
-                                      stderr=subprocess.STDOUT,
-                                      universal_newlines=True).strip()
-  except subprocess.CalledProcessError: # BBB: old GCC
-    current = subprocess.check_output(('gcc', '-dumpversion'),
-                                      universal_newlines=True).strip()
+  version_set = set()
+  for command in ('gcc', 'g++', 'gfortran'):
+    try:
+      version = subprocess.check_output((command, '-dumpfullversion'),
+                                        stderr=subprocess.STDOUT,
+                                        universal_newlines=True).strip()
+    except subprocess.CalledProcessError: # BBB: old GCC
+      version = subprocess.check_output((command, '-dumpversion'),
+                                        universal_newlines=True).strip()
+    except FileNotFoundError:
+      version = None
+    version_set.add(version)
+  if None in version_set or len(version_set) != 1:
+    current = '0.0.0'
+  else:
+    current, = version_set
   self.system_version = current
 # If we're still going to use the same GCC,
 # the conditions have no impact on the dependant parts.