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

CMakeLists.txt

@@ -204,7 +204,8 @@ endif()
 option(CURL_WERROR "Turn compiler warnings into errors" OFF)
 option(PICKY_COMPILER "Enable picky compiler options" ON)
 option(BUILD_CURL_EXE "Build curl executable" ON)
-option(BUILD_SHARED_LIBS "Build shared libraries" ON)
+get_property(_has_shared GLOBAL PROPERTY TARGET_SUPPORTS_SHARED_LIBS)
+option(BUILD_SHARED_LIBS "Build shared libraries" ${_has_shared})
 option(BUILD_STATIC_LIBS "Build static libraries" OFF)
 option(BUILD_STATIC_CURL "Build curl executable with static libcurl" OFF)
 option(ENABLE_ARES "Enable c-ares support" OFF)
@@ -260,7 +261,7 @@ if(WIN32)
     endif()
     unset(MINGW64_VERSION CACHE)  # Avoid storing in CMake cache
   endif()
-elseif(DOS OR AMIGA)
+elseif(DOS)
   set(BUILD_SHARED_LIBS OFF)
   set(BUILD_STATIC_LIBS ON)
 endif()