make compiler check fire only when needed

The compiler check in lsst.sconsUtils.state._configureCommon
fires whenever you import lsst.sconsUtils, even if you don't
have any C++ code and all you want out of lsst.sconsUtils is
some utility completely unrelated to C++. Besides just wasting
time, this makes it difficult to run ci_hsc on a cluster nodes
that don't have the latest gcc installed.

Instead, have the compiler check only fire when we need it, as
defined by instantiating BasicSConstruct. The compiler check
can also be disabled when using BasicSConstruct by instantiating
it with disableCc=True.

python/lsst/sconsUtils/scripts.py

@@ -60,9 +60,10 @@ def __new__(cls, packageName, versionString=None, eupsProduct=None, eupsProductP
                 defaultTargets=DEFAULT_TARGETS,
                 subDirList=None, ignoreRegex=None,
                 versionModuleName="python/lsst/%s/version.py", noCfgFile=False,
-                sconscriptOrder=None):
+                sconscriptOrder=None, disableCc=False):
         cls.initialize(packageName, versionString, eupsProduct, eupsProductPath, cleanExt,
-                       versionModuleName, noCfgFile=noCfgFile, sconscriptOrder=sconscriptOrder)
+                       versionModuleName, noCfgFile=noCfgFile, sconscriptOrder=sconscriptOrder,
+                       disableCc=disableCc)
         cls.finish(defaultTargets, subDirList, ignoreRegex)
         return state.env
 
@@ -99,7 +100,10 @@ def __new__(cls, packageName, versionString=None, eupsProduct=None, eupsProductP
     @classmethod
     def initialize(cls, packageName, versionString=None, eupsProduct=None, eupsProductPath=None,
                    cleanExt=None, versionModuleName="python/lsst/%s/version.py", noCfgFile=False,
-                   sconscriptOrder=None):
+                   sconscriptOrder=None, disableCc=False):
+        if not disableCc:
+            state._configureCommon()
+            state._saveState()
         if cls._initializing:
             state.log.fail("Recursion detected; an SConscript file should not call BasicSConstruct.")
         cls._initializing = True

python/lsst/sconsUtils/state.py

@@ -284,8 +284,16 @@ def _initEnvironment():
     env['eupsFlavor'] = eupsForScons.flavor()
 
 
+_configured = False
+
+
 def _configureCommon():
     """Configuration checks for the compiler, platform, and standard libraries."""
+    global _configured
+    if _configured:
+        return
+    _configured = True
+
     #
     # Is the C compiler really gcc/g++?
     #
@@ -503,7 +511,5 @@ def _saveState():
 _initLog()
 _initVariables()
 _initEnvironment()
-_configureCommon()
-_saveState()
 
 # @endcond