qt/5.x.x: Fix opengl/system dependency to not be required when using es2

The opengl/system dependency is very misleading.
For Linux, it is a requirement for the legacy gl interface used for X11.
This is not necessarily required when using the es2 OpenGL backend and likely Wayland.

This PR only requires opengl/system on Linux when with_x11 is enabled.

Fixes #22116.

recipes/qt/5.x.x/conanfile.py

@@ -384,9 +384,13 @@ def requirements(self):
             self.requires("libalsa/1.2.10")
         if self.options.get_safe("with_x11"):
             self.requires("xorg/system")
+            if self.options.get_safe("opengl", "no") != "no":
+                self.requires("opengl/system")
+        if self.settings.os in ["FreeBSD", "Linux"] and not self.options.get_safe("with_x11") and self.options.get_safe("opengl", "no") != "no":
+            self.requires("egl/system")
         if self.options.get_safe("with_x11") or self.options.qtwayland:
             self.requires("xkbcommon/1.5.0")
-        if self.options.get_safe("opengl", "no") != "no":
+        if self.options.get_safe("opengl", "no") != "no" and not self.settings.os in ["FreeBSD", "Linux"]:
             self.requires("opengl/system")
         if self.options.with_zstd:
             self.requires("zstd/1.5.5")
@@ -1077,7 +1081,9 @@ def _create_plugin(pluginname, libname, plugintype, requires):
                     gui_reqs.append("xkbcommon::xkbcommon")
                 if self.options.get_safe("with_x11", False):
                     gui_reqs.append("xorg::xorg")
-            if self.options.get_safe("opengl", "no") != "no":
+                    if self.options.get_safe("opengl", "no") != "no":
+                        gui_reqs.append("opengl::opengl")
+            if self.options.get_safe("opengl", "no") != "no" and self.settings.os not in ["FreeBSD", "Linux"]:
                 gui_reqs.append("opengl::opengl")
             if self.options.get_safe("with_vulkan", False):
                 gui_reqs.append("vulkan-loader::vulkan-loader")