Use == instead of is operator for checking string equality in AMBuild scripts (bug 6043, r=dvander).

core/logic/AMBuilder

@@ -14,9 +14,9 @@ binary.compiler.defines += [
   'SM_LOGIC'
 ]
   
-if builder.target_platform is 'linux':
+if builder.target_platform == 'linux':
   binary.compiler.postlink += ['-lpthread', '-lrt']
-elif builder.target_platform is 'mac':
+elif builder.target_platform == 'mac':
   binary.compiler.cflags += ['-Wno-deprecated-declarations']
   binary.compiler.postlink += ['-framework', 'CoreServices']
 binary.sources += [
@@ -64,7 +64,7 @@ binary.sources += [
   'smn_database.cpp',
   'ForwardSys.cpp',
 ]
-if builder.target_platform is 'windows':
+if builder.target_platform == 'windows':
   binary.sources += ['thread/WinThreads.cpp']
 else:
   binary.sources += ['thread/PosixThreads.cpp']