cmake: allow SOVERSION override with CURL_LIBCURL_SOVERSION

Allow overriding SOVERSION with the new CMake option:
`CURL_LIBCURL_SOVERSION=ON/OFF`

For certain target platforms the shared libcurl library filename
contains the SOVERSION. This new option allows to enable/disable
this behavior manually. If set, it takes precedence over the default
setting.

Ref: #13898
Closes #13944

Author: vszakats

lib/CMakeLists.txt

@@ -201,7 +201,14 @@ if(BUILD_SHARED_LIBS)
     # up to v3.x and ELF from v3.x. I cannot imagine someone running CMake
     # on those ancient systems.
     CMAKE_SYSTEM_NAME STREQUAL "FreeBSD")
+    set(soversion_default TRUE)
+  else()
+    set(soversion_default FALSE)
+  endif()
+
+  option(CURL_LIBCURL_SOVERSION "Enable libcurl SOVERSION" ${soversion_default})
 
+  if(CURL_LIBCURL_SOVERSION)
     transform_makefile_inc("Makefile.soname" "${CMAKE_CURRENT_BINARY_DIR}/Makefile.soname.cmake")
     include(${CMAKE_CURRENT_BINARY_DIR}/Makefile.soname.cmake)