Let Findglfw3.cmake detect whether the GLFW library is static, to pro…

…vide a fatal error when building a TGUI dll in such case instead of silently continuing and creating a broken dll file (closes #247)

cmake/Modules/Findglfw3.cmake

@@ -68,15 +68,23 @@ FIND_PACKAGE_HANDLE_STANDARD_ARGS(glfw3
 
 if(glfw3_FOUND)
   if(GLFW_LIBRARY AND NOT TARGET glfw)
-    add_library(glfw UNKNOWN IMPORTED)
-    set_target_properties(glfw PROPERTIES
-                          IMPORTED_LOCATION "${GLFW_LIBRARY}"
-                          INTERFACE_INCLUDE_DIRECTORIES "${GLFW_INCLUDE_DIR}")
-
     get_filename_component(GLFW_LIBRARY_FILENAME "${GLFW_LIBRARY}" NAME)
     if(GLFW_LIBRARY_FILENAME STREQUAL "glfw3dll.lib" OR GLFW_LIBRARY_FILENAME STREQUAL "libglfw3dll.a")
+        add_library(glfw UNKNOWN IMPORTED) # UNKNOWN instead of SHARED because we otherwise must also provide the path to the DLL on Windows
         set_target_properties(glfw PROPERTIES
                               INTERFACE_COMPILE_DEFINITIONS "GLFW_DLL")
+    elseif(CMAKE_SYSTEM_NAME STREQUAL "Windows"
+           AND (GLFW_LIBRARY_FILENAME STREQUAL "libglfw3.a"
+             OR GLFW_LIBRARY_FILENAME STREQUAL "glfw3.lib"
+             OR GLFW_LIBRARY_FILENAME STREQUAL "glfw3_mt.lib"))
+        # If we know that the library is static then explicitly mark it as such so that we can give an error when building TGUI as a dll
+        add_library(glfw STATIC IMPORTED)
+    else()
+        add_library(glfw UNKNOWN IMPORTED)
     endif()
+
+    set_target_properties(glfw PROPERTIES
+                          IMPORTED_LOCATION "${GLFW_LIBRARY}"
+                          INTERFACE_INCLUDE_DIRECTORIES "${GLFW_INCLUDE_DIR}")
   endif()
 endif()