cmake: Emulate Libtool's behavior on FreeBSD

[hebasto]

Building the master branch @ f24b838 on FreeBSD 14.3:

• with Autotools:

$ ./autogen.sh
$ ./configure --disable-static --prefix /tmp/AUTOTOOLS
$ gmake
$ gmake install
$ tree /tmp/AUTOTOOLS/lib
/tmp/AUTOTOOLS/lib
├── libsecp256k1.la
├── libsecp256k1.so -> libsecp256k1.so.5.0.1
├── libsecp256k1.so.5 -> libsecp256k1.so.5.0.1
├── libsecp256k1.so.5.0.1
└── pkgconfig
    └── libsecp256k1.pc

2 directories, 5 files

• with CMake:

$ cmake -B build -DCMAKE_INSTALL_PREFIX=/tmp/CMAKE
$ cmake --build build
$ cmake --install build
$ tree /tmp/CMAKE/lib
/tmp/CMAKE/lib
├── cmake
│   └── libsecp256k1
│       ├── libsecp256k1-config-version.cmake
│       ├── libsecp256k1-config.cmake
│       ├── libsecp256k1-targets-relwithdebinfo.cmake
│       └── libsecp256k1-targets.cmake
├── libsecp256k1.so -> libsecp256k1.so.5
├── libsecp256k1.so.5
└── pkgconfig
    └── libsecp256k1.pc

4 directories, 7 files

With this PR:

$ cmake -B build -DCMAKE_INSTALL_PREFIX=/tmp/CMAKE+PR
$ cmake --build build
$ cmake --install build
$ tree /tmp/CMAKE+PR/lib
/tmp/CMAKE+PR/lib
├── cmake
│   └── libsecp256k1
│       ├── libsecp256k1-config-version.cmake
│       ├── libsecp256k1-config.cmake
│       ├── libsecp256k1-targets-relwithdebinfo.cmake
│       └── libsecp256k1-targets.cmake
├── libsecp256k1.so -> libsecp256k1.so.5
├── libsecp256k1.so.5 -> libsecp256k1.so.5.0.1
├── libsecp256k1.so.5.0.1
└── pkgconfig
    └── libsecp256k1.pc

4 directories, 8 files

From FreeBSD Developers' Handbook:

If you are adding shared library support to a port or other piece of software that does not have one, the version numbers should follow these rules. Generally, the resulting numbers will have nothing to do with the release version of the software.

For ports:

• Prefer using the number already selected by upstream

• If upstream provides symbol versioning, ensure that we use their script

[real-or-random]

utACK 37dd422

[real-or-random]

What do you think about the alternative version of making this the else branch if neither on Windows nor on macOS?

[hebasto]

What do you think about the alternative version of making this the else branch if neither on Windows nor on macOS?

That was my initial thought too. However, shared library naming conventions differ on other systems, such as OpenBSD and NetBSD. So I chose the current approach to self‑document which systems are tested.

[hebasto]

What do you think about the alternative version of making this the else branch if neither on Windows nor on macOS?

That was my initial thought too. However, shared library naming conventions differ on other systems, such as OpenBSD and NetBSD. So I chose the current approach to self‑document which systems are tested.

I've included a reference to the FreeBSD docs in the PR description.