Make tests build in a separate folder, add to gitignore

tests/SConstruct

@@ -26,7 +26,7 @@ if 'CXXFLAGS' in ARGUMENTS: env.Append(CCFLAGS = ARGUMENTS['CXXFLAGS'])
 add_nanopb_builders(env)
 
 # Path to the files shared by tests, and to the nanopb core.
-env.Append(CPPPATH = ["#../", "#common"])
+env.Append(CPPPATH = ["#../", "$COMMON"])
 
 # Path for finding nanopb.proto
 env.Append(PROTOCPATH = '#../generator')
@@ -110,5 +110,10 @@ elif 'cl' in env['CXX']:
     env.Append(CXXFLAGS = '/Zi /W2 /WX')
     
 # Now include the SConscript files from all subdirectories
-SConscript(Glob('*/SConscript'), exports = 'env')
+import os.path
+env['VARIANT_DIR'] = 'build'
+env['BUILD'] = '#' + env['VARIANT_DIR']
+env['COMMON'] = '#' + env['VARIANT_DIR'] + '/common'
+for subdir in Glob('*/SConscript'):
+    SConscript(subdir, exports = 'env', variant_dir = env['VARIANT_DIR'] + '/' + os.path.dirname(str(subdir)))