cmake: automatic exports macro tidy-up (#875)

In a recent CMake update I left the original CMake EXPORTS macro
unchanged (`libssh2_EXPORTS`) for compatibility.

However, that macro was also recently added [1] and not present in an
official release yet, so we might as well just use the new native one
instead (`libssh2_shared_EXPORTS`), defined by CMake automatically.
This way we don't need to define the old macro manually.

CMake forms this macro from the lib's internal name as defined in
`add_library()` by appending `_EXPORTS`. That target name changed from
`libssh2` to `libssh2_shared` after introducing dual shared + static
builds in the recent update.

If we're here, add a new, stable, build-tool agnostic macro with the
same effect, for non-CMake use: `LIBSSH2_EXPORTS`

[1] 1f0fe7443a (2021-10-26)

Follow-up to 4e2580628d

include/libssh2.h

@@ -100,7 +100,8 @@ extern "C" {
 /* Allow alternate API prefix from CFLAGS or calling app */
 #ifndef LIBSSH2_API
 # ifdef WIN32
-#  if defined(_WINDLL) || defined(libssh2_EXPORTS)
+#  if defined(LIBSSH2_EXPORTS) || \
+      defined(_WINDLL) || defined(libssh2_shared_EXPORTS)
 #   ifdef LIBSSH2_LIBRARY
 #    define LIBSSH2_API __declspec(dllexport)
 #   else