gcc 11 /CryptoPP.{cc,h} breakage, and workaround with CXXFLAGS=-std=c++14

[HinTak]

gcc 11 defaults to c++17, which introduces std::byte, which causes a lot of breakage when compiling src/utils/lib/CryptoPP.hsrc/utils/lib/CryptoPP.{cc,h} .

Apparently the problem has been anticipated upstream,
https://cryptopp.com/wiki/Std::byte , and somewhat address with cryptopp 6.x . (while mldonkey has 5. x according to the header). For the time being, doing CXXFLAGS=-std=c++14 works; but it is probably best upgrading...

[HinTak]

Since it already checks g++ version and try a few flags

mldonkey/config/configure.in

Line 1180 in 333868a

ACX_CHECK_CXX_FLAGS(-fno-omit-frame-pointer,cv_no_omit_frame_pointer, CRYPTOPPFLAGS="-fno-omit-frame-pointer")

, it may be a good idea to include ...std=c++14 in any case?

[HinTak]

Pull in #63 as outlined

[HinTak]

#63 merged. Can close.