CMake: Avoid target_link_libraries() with imported targets

Versions of CMake prior to 3.11 do not support using
target_link_libraries() to setup INTERFACE_LINK_LIBRARIES.
Using set_property() directly works in all 3.x versions,
including 3.11 and newer.

Fixes #75

(cherry picked from commit c06f791)

cmake/FindEGL.cmake

@@ -51,7 +51,8 @@ mark_as_advanced(EGL_INCLUDE_DIR EGL_LIBRARY)
 if (EGL_LIBRARIES AND NOT TARGET GL::egl)
     add_library(GL::egl INTERFACE IMPORTED)
     if (TARGET PkgConfig::EGL)
-        target_link_libraries(GL::egl INTERFACE PkgConfig::EGL)
+        set_property(TARGET GL::egl PROPERTY
+            INTERFACE_LINK_LIBRARIES PkgConfig::EGL)
     else ()
         set_property(TARGET GL::egl PROPERTY
             INTERFACE_LINK_LIBRARIES ${EGL_LIBRARY})

cmake/FindWPE.cmake

@@ -48,7 +48,8 @@ mark_as_advanced(WPE_INCLUDE_DIR WPE_LIBRARY)
 if (WPE_LIBRARY AND NOT TARGET WPE::libwpe)
     add_library(WPE::libwpe INTERFACE IMPORTED)
     if (TARGET PkgConfig::WPE)
-        target_link_libraries(WPE::libwpe INTERFACE PkgConfig::WPE)
+        set_property(TARGET WPE::libwpe PROPERTY
+            INTERFACE_LINK_LIBRARIES PkgConfig::WPE)
     else ()
         set_property(TARGET WPE::libwpe PROPERTY
             INTERFACE_LINK_LIBRARIES "${WPE_LIBRARY}")