Address PR remarks

Author: victorusu

config/cscs.py

@@ -534,8 +534,8 @@ class ReframeSettings:
 
                 'builtin': {
                     'cc':  'cc',
-                    'cxx': '',
-                    'ftn': '',
+                    'cxx': 'CC',
+                    'ftn': 'ftn',
                 },
 
                 'builtin-gcc': {

cscs-checks/libraries/boost/boost_python_check.py

@@ -14,7 +14,7 @@ def __init__(self, boost_version, cray_gnu_version, python_version):
                       'support') % (boost_version, cray_gnu_version,
                                     python_version)
         self.valid_systems = ['daint:mc', 'daint:gpu', 'dom:mc', 'dom:gpu']
-        self.valid_prog_environs = ['PrgEnv-gnu']
+        self.valid_prog_environs = ['builtin']
         python_major, python_minor = python_version.split('.')
         self.modules = ['Boost/%s-CrayGNU-%s-python%s' % (
             boost_version, cray_gnu_version, python_major)]

cscs-checks/libraries/gridtools/gridtools_tests.py

@@ -18,7 +18,7 @@ class GridToolsCheck(rfm.RegressionTest):
     def __init__(self, variant):
         # Check if this is a device check
         self.descr = 'GridTools test base'
-        self.valid_prog_environs = ['PrgEnv-gnu']
+        self.valid_prog_environs = ['builtin']
         self.modules = ['CMake', 'Boost']
         is_cuda_test = 'cuda' in variant
         if is_cuda_test:

cscs-checks/libraries/magma/magma_checks.py

@@ -18,7 +18,7 @@ def __init__(self, subtest):
 
         self.prebuild_cmd = ['patch < patch.txt']
         self.build_system = 'Make'
-        self.valid_prog_environs = ['PrgEnv-intel']
+        self.valid_prog_environs = ['builtin']
         self.build_system.makefile = 'Makefile_%s' % subtest
         # Compile with -O0 since with a higher level the compiler seems to
         # optimise something away