cmake: link libOpenGL instead of legacy libGL (#44)

From cmake documentation, Linux-specific chapter:

  Projects may use the ``OpenGL::GL`` target (or ``OPENGL_LIBRARIES``
  variable) to use legacy GL interfaces.  These will use the legacy GL
  library located by ``OPENGL_gl_LIBRARY``, if available.  If
  ``OPENGL_gl_LIBRARY`` is empty or not found and GLVND is available,
  the ``OpenGL::GL`` target will use GLVND ``OpenGL::OpenGL`` and
  ``OpenGL::GLX`` (and the ``OPENGL_LIBRARIES`` variable will use the
  corresponding libraries).  Thus, for non-EGL-based Linux targets, the
  ``OpenGL::GL`` target is most portable.

which means linking with OpenGL::GL makes cmake find legacy libGL.so or
GLX libraries, and as for now, the former contains GLX symbols and cannot
be built without X libraries. Since we are working with EGL, it wouldn't
provide extra portability, either.

This patch switches to link aquamarine with modern OpenGL::OpenGL, which
contains core OpenGL API only, makes it possible to build on a
wayland-only system. Tested on eweOS, which is a distro without X
libraries.

Link: https://os.ewe.moe/

Signed-off-by: Yao Zi <ziyao@disroot.org>

CMakeLists.txt

@@ -60,7 +60,7 @@ target_include_directories(
   PRIVATE "./src" "./src/include" "./protocols" "${CMAKE_BINARY_DIR}")
 set_target_properties(aquamarine PROPERTIES VERSION ${AQUAMARINE_VERSION}
                                             SOVERSION 2)
-target_link_libraries(aquamarine OpenGL::EGL OpenGL::GL PkgConfig::deps)
+target_link_libraries(aquamarine OpenGL::EGL OpenGL::OpenGL PkgConfig::deps)
 
 check_include_file("sys/timerfd.h" HAS_TIMERFD)
 pkg_check_modules(epoll IMPORTED_TARGET epoll-shim)