This also means that we no longer pass any picky warning option to
ossfuzz. It's probably not worth maintaining picky C++ options for this
single, small target.
Silencing:
```
cc1plus: warning: command-line option '-Wbad-function-cast' is valid for C/ObjC but not for C++
cc1plus: warning: command-line option '-Wdeclaration-after-statement' is valid for C/ObjC but not for C++
cc1plus: warning: command-line option '-Wenum-int-mismatch' is valid for C/ObjC but not for C++
cc1plus: warning: command-line option '-Wjump-misses-init' is valid for C/ObjC but not for C++
cc1plus: warning: command-line option '-Wmissing-parameter-type' is valid for C/ObjC but not for C++
cc1plus: warning: command-line option '-Wmissing-prototypes' is valid for C/ObjC but not for C++
cc1plus: warning: command-line option '-Wnested-externs' is valid for C/ObjC but not for C++
cc1plus: warning: command-line option '-Wold-style-declaration' is valid for C/ObjC but not for C++
cc1plus: warning: command-line option '-Wold-style-definition' is valid for C/ObjC but not for C++
cc1plus: warning: command-line option '-Wstrict-prototypes' is valid for C/ObjC but not for C++
```
Ref: https://github.com/libssh2/libssh2/actions/runs/18063134305/job/51402236388#step:7:15Closes#1686