GCC 15 fails to compile cpluff::less_str

[gruenich]

openSuse package build service uses GCC 15. c-pluff no longer compiles due to the following error:

[   25s] /usr/include/c++/15/bits/stl_tree.h:2604:36: error: no match for call to '(const cpluff::less_str) (const char* const&, const char* const&)'
[   25s]  2604 |         if (!_M_impl._M_key_compare(_S_key(__x), __k))
[   25s]       |              ~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~
[   25s] In file included from ./cpluffxx.h:42:
[   25s] ./cpluffxx/info.h:44:7: note: there is 1 candidate
[   25s]    44 | class less_str {
[   25s]       |       ^~~~~~~~
[   25s] ./cpluffxx/info.h:54:21: note: candidate 1: 'bool cpluff::less_str::operator()(const char* const&, const char* const&)' (near match)
[   25s]    54 |         inline bool operator()(const char* const& s1, const char* const& s2) {
[   25s]       |                     ^~~~~~~~
[   25s] ./cpluffxx/info.h:54:21: note: passing 'const cpluff::less_str*' as 'this' argument discards qualifiers

[jlehtine]

#12 makes the C++ API compatible with C++17 and allows compilation with GCC 15 (tried with 15.2.0).

C++ testsuite still fails but I am not sure if it has passed before. Will be looking into it.