Fix bug with getConfigFileFromTac() and relocatable buildmasters.

On lines 47 - 48 of getConfigFileFromTac(), tacGlobals is initialized as an empty dict and passed to execfile as an argument when loading the *.tac file from a buildmaster. However, relocatable buildmasters rely on the __file__ attribute to be set, which does not happen and causes an exception. This patch sets __file__ in tacGlobals before calling execfile.