compilation error in 'directory_name' and 'basename'

[Peter Johansson]

get the following error on Mac OS 10.6:

g++ -DHAVE_CONFIG_H -I. -I../../lib -I.. -I../yat  -I../.. -DNDEBUG -I/opt/local/include/apr-1 -DDARWIN -DSIGPROCMASK_SETS_THREAD_MASK -no-cpp-precomp -DDARWIN_10 -I/opt/local/include  -O3 -Wall -MT utility.o -MD -MP -MF .deps/utility.Tpo -c -o utility.o ../../lib/utility.cc
../../lib/utility.cc: In function 'std::string theplu::svndigest::directory_name(std::string)':
../../lib/utility.cc:121: error: invalid conversion from 'const char*' to 'char*'
../../lib/utility.cc:121: error:   initializing argument 1 of 'char* dirname(char*)'
../../lib/utility.cc: In function 'std::string theplu::svndigest::file_name(const std::string&)':
../../lib/utility.cc:127: error: invalid conversion from 'const char*' to 'char*'
../../lib/utility.cc:127: error:   initializing argument 1 of 'char* basename(char*)'
make[2]: *** [utility.o] Error 1

[Peter Johansson]

(In [1432]) implement file_name and directory_name. fixes #511