Thorough CMakeLists cleanup.

* Explicitly installing header files instead of recursive glob (some
   header are internal, thus no need to install them).
 * Calling enable_testing() only in project root.
 * Using MAGNUM_LIBRARIES instead of specifying all dependencies
   explicitly.
 * Using CORRADE_CXX_FLAGS instead of our own (heavily outdated) set.
 * Removed FindMagnumPlugins.cmake, the module does essentially nothing.
 * Various reorganization and cleanup.

src/Plugins/TgaImporter/CMakeLists.txt

@@ -1,16 +1,22 @@
-add_library(TgaImporterObjects OBJECT TgaImporter.cpp)
+set(TgaImporter_SRCS
+    TgaImporter.cpp)
+
+set(TgaImporter_HEADERS
+    TgaImporter.h)
+
+add_library(TgaImporterObjects OBJECT ${TgaImporter_SRCS})
 set_target_properties(TgaImporterObjects PROPERTIES COMPILE_FLAGS ${CMAKE_SHARED_LIBRARY_CXX_FLAGS})
 
-corrade_add_plugin(TgaImporter ${MAGNUM_PLUGINS_IMPORTER_INSTALL_DIR} TgaImporter.conf $<TARGET_OBJECTS:TgaImporterObjects> pluginRegistrationTgaImporter.cpp)
-target_link_libraries(TgaImporter ${MAGNUM_LIBRARY})
+corrade_add_plugin(TgaImporter ${MAGNUM_PLUGINS_IMPORTER_INSTALL_DIR}
+    TgaImporter.conf
+    $<TARGET_OBJECTS:TgaImporterObjects>
+    pluginRegistrationTgaImporter.cpp)
+target_link_libraries(TgaImporter ${MAGNUM_LIBRARIES})
+
+install(FILES ${TgaImporter_HEADERS} DESTINATION ${MAGNUM_PLUGINS_INCLUDE_INSTALL_DIR}/TgaImporter)
 
 if(BUILD_TESTS)
     add_library(TgaImporterTestLib $<TARGET_OBJECTS:TgaImporterObjects>)
-    target_link_libraries(TgaImporterTestLib ${MAGNUM_LIBRARY} ${CORRADE_UTILITY_LIBRARY} ${CORRADE_PLUGINMANAGER_LIBRARY})
-    enable_testing()
+    target_link_libraries(TgaImporterTestLib ${MAGNUM_LIBRARIES})
     add_subdirectory(Test)
 endif()
-
-if(WIN32)
-    target_link_libraries(TgaImporter ${CORRADE_UTILITY_LIBRARY} ${CORRADE_PLUGINMANAGER_LIBRARY})
-endif()