Require ObjCryst when enable_objcryst is set.

Exit with build error if enable_objcryst is true and ObjCryst cannot
be found.  Use conditional linking with ObjCryst if enable_objcryst
was not set.

Author: pavoljuhas

SConstruct

@@ -11,7 +11,7 @@ install-include     install the C++ header files
 install-data        install data files used by the library
 alltests            build the unit test program "alltests"
 test                execute unit tests (requires the cxxtest framework)
-sdist               create source distribution tarball (requires git repo)
+sdist               create source distribution tarball from git repository
 
 Build configuration variables:
 %s
@@ -68,7 +68,7 @@ vars.Add(PathVariable('datadir',
     env['prefix'] + '/share',
     PathVariable.PathAccept))
 vars.Add(BoolVariable('enable_objcryst',
-    'enable objcryst support, when installed', True))
+    'enable objcryst support, when installed', None))
 vars.Update(env)
 env.Help(MY_SCONS_HELP % vars.GenerateHelpText(env))
 

src/SConscript.configure

@@ -55,10 +55,18 @@ if not conf.CheckBoostVersion(boost_required):
 # boost_serialization
 configure_boost_library('boost_serialization')
 
-# check for ObjCryst, autoadd appends to LIBS if found.
-conf.env['has_objcryst'] = (conf.env['enable_objcryst'] and
-    conf.CheckLibWithHeader('ObjCryst', 'ObjCryst/ObjCryst/Crystal.h',
-        language='C++', autoadd=True))
+# ObjCryst - assume a no-objcryst fallback configuration.
+conf.env['has_objcryst'] = False
+# Detect ObjCryst and exit with error if requested and not found.
+# By default conf.env does not contain the 'enable_objcryst' key.
+if conf.env.get('enable_objcryst', True):
+    conf.env['has_objcryst'] = conf.CheckLibWithHeader(
+        'ObjCryst', 'ObjCryst/ObjCryst/Crystal.h',
+        language='C++', autoadd=True)
+    objcryst_requested = conf.env.get('enable_objcryst', False)
+    if objcryst_requested and not conf.env['has_objcryst']:
+        print('Adjust compiler paths or build with "enable_objcryst=False".')
+        Exit(1)
 
 env = conf.Finish()