cmake: adjust defaults for target platforms not supporting shared libs

If CMake reports the target platform not supporting shared libs, turn
`BUILD_SHARED_LIBS` off by default. CMake 3.30+ fails with an error
when trying to create a `SHARED` target for such platforms. Earlier
versions used a workaround that may or may not have worked in practice.

Ref: https://cmake.org/cmake/help/v3.30/policy/CMP0164.html

Seen this with a build setting `-DCMAKE_SYSTEM_NAME=Generic`, e.g.
AmigaOS.

Note this may introduce incompatibility for "Generic" targets, which
support shared libs. If that's the case, set `BUILD_SHARED_LIBS=ON`
manually.

Also drop AmigaOS-specific logic handled automatically after this patch.

Ref: https://cmake.org/cmake/help/v3.7/command/get_property.html
Ref: https://cmake.org/cmake/help/v3.7/prop_gbl/TARGET_SUPPORTS_SHARED_LIBS.html

Closes #19420

docs/INSTALL-CMAKE.md

@@ -221,7 +221,7 @@ target_link_libraries(my_target PRIVATE CURL::libcurl)
 - `BUILD_EXAMPLES`:                         Build libcurl examples. Default: `ON`
 - `BUILD_LIBCURL_DOCS`:                     Build libcurl man pages. Default: `ON`
 - `BUILD_MISC_DOCS`:                        Build misc man pages (e.g. `curl-config` and `mk-ca-bundle`). Default: `ON`
-- `BUILD_SHARED_LIBS`:                      Build shared libraries. Default: `ON`
+- `BUILD_SHARED_LIBS`:                      Build shared libraries. Default: `ON` (if target platform supports shared libs, otherwise `OFF`)
 - `BUILD_STATIC_CURL`:                      Build curl executable with static libcurl. Default: `OFF` (turns to `ON`, when building static libcurl only)
 - `BUILD_STATIC_LIBS`:                      Build static libraries. Default: `OFF` (turns to `ON` if `BUILD_SHARED_LIBS` is `OFF`)
 - `BUILD_TESTING`:                          Build tests. Default: `ON`