cmake: (re-)add aliases for add_subdirectory() builds

Add internal libssh2 library aliases to make these available for
downstream/dependent projects building libssh2 via `add_subdirectory()`:

- `libssh2:libssh2_static`
- `libssh2:libssh2_shared`
- `libssh2:libssh2` (shared, or static when not building shared)
- `libssh2` (shared, or static when not building shared)

Of these, `libssh2` was present in v1.10.0 and earlier releases, but
missing from v1.11.0.

Closes #1169

src/CMakeLists.txt

@@ -106,6 +106,7 @@ endif()
 if(BUILD_STATIC_LIBS)
   list(APPEND libssh2_export ${LIB_STATIC})
   add_library(${LIB_STATIC} STATIC ${SOURCES})
+  add_library(${PROJECT_NAME}::${LIB_STATIC} ALIAS ${LIB_STATIC})
   target_compile_definitions(${LIB_STATIC} PRIVATE ${PRIVATE_COMPILE_DEFINITIONS} ${libssh2_DEFINITIONS})
   target_link_libraries(${LIB_STATIC} PRIVATE ${LIBRARIES})
   set_target_properties(${LIB_STATIC} PROPERTIES
@@ -121,6 +122,7 @@ endif()
 if(BUILD_SHARED_LIBS)
   list(APPEND libssh2_export ${LIB_SHARED})
   add_library(${LIB_SHARED} SHARED ${SOURCES})
+  add_library(${PROJECT_NAME}::${LIB_SHARED} ALIAS ${LIB_SHARED})
   if(WIN32)
     set_property(TARGET ${LIB_SHARED} APPEND PROPERTY SOURCES "libssh2.rc")
   endif()
@@ -139,6 +141,9 @@ if(BUILD_SHARED_LIBS)
       $<INSTALL_INTERFACE:$<INSTALL_PREFIX>/${CMAKE_INSTALL_INCLUDEDIR}>)
 endif()
 
+add_library(${PROJECT_NAME}::${LIB_NAME} ALIAS ${LIB_SELECTED})
+add_library(${LIB_NAME} ALIAS ${LIB_SELECTED})
+
 ## Installation
 
 install(FILES