File build20-libGL.patch of Package widelands

diff -ur widelands-build20/CMakeLists.txt widelands-build20_fix/CMakeLists.txt
--- widelands-build20/CMakeLists.txt	2019-05-02 07:23:39.000000000 +0200
+++ widelands-build20_fix/CMakeLists.txt	2019-07-10 21:43:46.945029296 +0200
@@ -181,7 +181,7 @@
 
 # This is set to avoid linker errors when using GLVND-libs on Linux
 if("${OpenGL_GL_PREFERENCE}" STREQUAL "GLVND")
-   set (CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} -lGL")
+   link_libraries("GL")
    add_compile_definitions(WL_USE_GLVND)
    message(STATUS "Adding linker flags for GLVND.")
 endif()