#18 Patch to improve compatibility with C++11

[Martin Gieseking]

The attached patch fixes to issues that prevent to build potrace with a C++11 compiler:
functions returning string constants (like potrace_version) should be of type const char* rather than const*
additional spaces are required when concatenating string constants and string macros (due to the new concept of string literals)

[Peter Selinger]

Thansk! Will be fixed in the next release.