bugfix: pgcc -V returns 2 on power machines

`pgcc -V` was failing on power machines because it returns 2 (despite
correctly printing version information).  On x86_64 machines the same
command returns 0 and doesn't cause an error.

- [x] Ignore return value of 2 for pgcc when doign a version check

lib/spack/spack/compiler.py

@@ -32,7 +32,7 @@ def _verify_executables(*paths):
 
 
 @llnl.util.lang.memoized
-def get_compiler_version_output(compiler_path, version_arg):
+def get_compiler_version_output(compiler_path, version_arg, ignore_errors=()):
     """Invokes the compiler at a given path passing a single
     version argument and returns the output.
 
@@ -41,7 +41,8 @@ def get_compiler_version_output(compiler_path, version_arg):
         version_arg (str): the argument used to extract version information
     """
     compiler = spack.util.executable.Executable(compiler_path)
-    output = compiler(version_arg, output=str, error=str)
+    output = compiler(
+        version_arg, output=str, error=str, ignore_errors=ignore_errors)
     return output
 
 
@@ -199,6 +200,9 @@ class Compiler(object):
     #: Compiler argument that produces version information
     version_argument = '-dumpversion'
 
+    #: Return values to ignore when invoking the compiler to get its version
+    ignore_version_errors = ()
+
     #: Regex used to extract version from compiler's output
     version_regex = '(.*)'
 
@@ -412,7 +416,8 @@ def c11_flag(self):
     @classmethod
     def default_version(cls, cc):
         """Override just this to override all compiler version functions."""
-        output = get_compiler_version_output(cc, cls.version_argument)
+        output = get_compiler_version_output(
+            cc, cls.version_argument, tuple(cls.ignore_version_errors))
         return cls.extract_version_from_output(output)
 
     @classmethod

lib/spack/spack/compilers/pgi.py

@@ -30,6 +30,7 @@ class Pgi(Compiler):
     PrgEnv_compiler = 'pgi'
 
     version_argument = '-V'
+    ignore_version_errors = [2]  # `pgcc -V` on PowerPC annoyingly returns 2
     version_regex = r'pg[^ ]* ([0-9.]+)-[0-9]+ (LLVM )?[^ ]+ target on '
 
     @classmethod