Avoid duplicated initialization of LIB_INSTALL_DIR

LIB_INSTALL_DIR is already initialized with the relative path `lib`, that works fine when used with the [CMake's `install` command](https://cmake.org/cmake/help/v3.0/command/install.html).

CMakeLists.txt

@@ -15,8 +15,6 @@ set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${CMAKE_BINARY_DIR}/bin)
 
-set(LIB_INSTALL_DIR ${CMAKE_INSTALL_PREFIX}/lib CACHE PATH "Installation path of libraries")
-
 ###########
 # Version #
 ###########
@@ -26,6 +24,7 @@ set(OPEN62541_VER_MINOR 3)
 set(OPEN62541_VER_PATCH 0)
 set(OPEN62541_VER_LABEL "dev") # Appended to the X.Y.Z version format. For example "-rc1" or an empty string
 
+#  If a relative path is specified, it is treated as relative to the $<INSTALL_PREFIX>
 set(LIB_INSTALL_DIR lib CACHE PATH "Installation path of libraries")
 
 # Set OPEN62541_VER_COMMIT