undef symbols to avoid warning

[gerkey]

undef symbols before defining them to avoid preprocessor warnings in the case that they were already defined.

I'm currently only getting only getting a warning on one symbol:

/home/gerkey/code/rospack-caching/src/rospack/src/rospack.cpp:78:0: warning: "PyUnicode_FromString" redefined [enabled by default]
 #define PyUnicode_FromString PyString_FromString
 ^
In file included from /usr/include/python2.7/Python.h:85:0,
                 from /home/gerkey/code/rospack-caching/src/rospack/src/rospack.cpp:72:
/usr/include/python2.7/unicodeobject.h:281:0: note: this is the location of the previous definition
 # define PyUnicode_FromString PyUnicodeUCS4_FromString
 ^

but we might as well undef them all.

[dirk-thomas]

+1