cmake: enable shared libssh2 library by default

This brings default behaviour in sync with autotools, which builds both
lib flavours by default.

(Notice that on Windows, autotools includes the Windows Resource in the
static library, when building both at the same time. CMake doesn't have
this issue.)

Enabling both lib flavours has a side-effect when using non-MinGW
toolchains (e.g. MSVC): to resolve the filename conflict between import
and static libraries, we add a suffix to the static lib, naming it
`libssh2_static.lib`. This can break dependent builds relying on
`libssh2.lib` for linking the static libssh2.

Workarounds:

- disable either shared or static libssh2 via
  `-DBUILD_STATIC_LIBS=OFF` or
  `-DBUILD_SHARED_LIBS=OFF`. This results in a libssh2 library (either
  static or shared) without a prefix: `libssh2.lib`.

- set a custom static library suffix via:
  `-DSTATIC_LIB_SUFFIX=_my_static`. Resulting in
  `libssh2_my_static.lib`, and import library
  `libssh2.lib`.

- set a custom import library suffix via:
  `-DIMPORT_LIB_SUFFIX=_my_implib`. Resulting in
  `libssh2_my_implib.lib` import library, and static library
  `libssh2.lib`.

- customize the default static/import library suffix (incl. extension)
  via
  `-DCMAKE_STATIC_LIBRARY_SUFFIX=_my_static_suffix.lib` or
  `-DCMAKE_IMPORT_LIBRARY_SUFFIX=_my_import_suffix.lib`.

Cherry-picked from #1036

CMakeLists.txt

@@ -56,7 +56,7 @@ option(BUILD_STATIC_LIBS "Build Static Libraries" ON)
 add_feature_info("Static library" BUILD_STATIC_LIBS
   "creating libssh2 static library")
 
-option(BUILD_SHARED_LIBS "Build Shared Libraries" OFF)
+option(BUILD_SHARED_LIBS "Build Shared Libraries" ON)
 add_feature_info("Shared library" BUILD_SHARED_LIBS
   "creating libssh2 shared library (.so/.dll)")