[c][cpp] Don't set CMake install dir to "./", closes #2320.

CMakeLists.txt

@@ -1,7 +1,9 @@
 cmake_minimum_required(VERSION 3.17)
 project(spine)
 
-set(CMAKE_INSTALL_PREFIX "./")
+if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
+	set(CMAKE_INSTALL_PREFIX ${CMAKE_BINARY_DIR} CACHE PATH "Install location" FORCE)
+endif()
 set(CMAKE_VERBOSE_MAKEFILE ON)
 set(SPINE_SFML FALSE CACHE BOOL FALSE)
 set(SPINE_SANITIZE FALSE CACHE BOOL FALSE)