Allow build to proceed with failed compiler lib detection (#12756)

Fixes #12732
Fixes #12767

c22a145 added automatic detection and RPATHing of compiler libraries
to Spack builds. However, in cases where the parsing/detection logic
fails this was terminating the build. This makes the compiler library
detection "best-effort" and reports an issue when the detection fails
rather than terminating the build.

lib/spack/spack/compiler.py

@@ -294,6 +294,10 @@ def determine_implicit_rpaths(cls, paths):
                                       output=str, error=str))  # str for py2
 
             return cls.parse_implicit_rpaths(output)
+        except spack.util.executable.ProcessError as pe:
+            tty.debug('ProcessError: Command exited with non-zero status: ' +
+                      pe.long_message)
+            return []
         finally:
             shutil.rmtree(tmpdir, ignore_errors=True)