Reuse already compiled objects for unit test libraries.

Speeds up compilation like infinity times. CMake 2.8.8 is required for
OBJECT library target.

src/Plugins/TgaImporter/CMakeLists.txt

@@ -1,8 +1,13 @@
-corrade_add_plugin(TgaImporter ${MAGNUM_PLUGINS_IMPORTER_INSTALL_DIR} TgaImporter.conf TgaImporter.cpp)
+add_library(TgaImporterObjects OBJECT TgaImporter.cpp)
+if(USE_FPIC)
+    set_target_properties(TgaImporterObjects PROPERTIES COMPILE_FLAGS -fPIC)
+endif()
+
+corrade_add_plugin(TgaImporter ${MAGNUM_PLUGINS_IMPORTER_INSTALL_DIR} TgaImporter.conf $<TARGET_OBJECTS:TgaImporterObjects>)
 target_link_libraries(TgaImporter ${MAGNUM_LIBRARY})
 
 if(BUILD_TESTS)
-    add_library(TgaImporterTestLib TgaImporter.cpp)
+    add_library(TgaImporterTestLib $<TARGET_OBJECTS:TgaImporterObjects>)
     target_link_libraries(TgaImporterTestLib ${MAGNUM_LIBRARY} ${CORRADE_UTILITY_LIBRARY} ${CORRADE_PLUGINMANAGER_LIBRARY})
     enable_testing()
     add_subdirectory(Test)