Add C++ / C compatibility code.
Add C++ / C compatibility code.

* include/private/svn_adler32.h: 
  under C++, mark our declarations as 'extern "C"'

jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion,jmckaskill/subversion