Fix breakage of non-GLES2 setups (#8774)

The commit 526a9e4 breaks
the non-GLES2 setups because the code that is intended to
handle that is behind "elseif()" which is interpreted as
"elseif(false)" and thus the code never gets executed. Fix
that by changing the offending line to else().

Additionally, to avoid breaking the server only build
(which shall not have a dependency on GL/GLU/GLES at all),
enclose the entire block code in if(BUILD_CLIENT).

Author: osjc

src/CMakeLists.txt

@@ -104,15 +104,17 @@ endif()
 
 option(ENABLE_GLES "Use OpenGL ES instead of OpenGL" FALSE)
 mark_as_advanced(ENABLE_GLES)
-if(ENABLE_GLES)
-	find_package(OpenGLES2 REQUIRED)
-elseif()
-	if(NOT WIN32) # Unix probably
-		set(OPENGL_GL_PREFERENCE "LEGACY" CACHE STRING
-			"See CMake Policy CMP0072 for reference. GLVND is broken on some nvidia setups")
-		set(OpenGL_GL_PREFERENCE ${OPENGL_GL_PREFERENCE})
-
-		find_package(OpenGL REQUIRED)
+if(BUILD_CLIENT)
+	if(ENABLE_GLES)
+		find_package(OpenGLES2 REQUIRED)
+	else()
+		if(NOT WIN32) # Unix probably
+			set(OPENGL_GL_PREFERENCE "LEGACY" CACHE STRING
+				"See CMake Policy CMP0072 for reference. GLVND is broken on some nvidia setups")
+			set(OpenGL_GL_PREFERENCE ${OPENGL_GL_PREFERENCE})
+
+			find_package(OpenGL REQUIRED)
+		endif()
 	endif()
 endif()