CMake: finding libssh2 should be idempotent

With the current code, running 'cmake .' in an already-configured
directory causes the removal of ssh flags passed to the compiler,
making it impossible to build with ssh support but by removing CMake's
cache.

Remove the check for LIBSSH2_LIBRARY and let CMake do the right thing
wrt finding the library.

CMakeLists.txt

@@ -148,15 +148,14 @@ ELSE()
 	FILE(GLOB SRC_ZLIB deps/zlib/*.c deps/zlib/*.h)
 ENDIF()
 
-IF(NOT LIBSSH2_LIBRARY)
-	FIND_PACKAGE(LIBSSH2 QUIET)
-ENDIF()
+FIND_PACKAGE(LIBSSH2 QUIET)
 IF (LIBSSH2_FOUND)
 	ADD_DEFINITIONS(-DGIT_SSH)
 	INCLUDE_DIRECTORIES(${LIBSSH2_INCLUDE_DIR})
 	SET(SSH_LIBRARIES ${LIBSSH2_LIBRARIES})
 ENDIF()
 
+
 # Platform specific compilation flags
 IF (MSVC)