Fixed visibility macros for plugins used as dependencies.

The symbols weren't exported on mingw32, causing linker errors.

src/Plugins/TgaImageConverter/CMakeLists.txt

@@ -29,7 +29,7 @@ set(TgaImageConverter_HEADERS
     TgaImageConverter.h)
 
 add_library(TgaImageConverterObjects OBJECT ${TgaImageConverter_SRCS})
-set_target_properties(TgaImageConverterObjects PROPERTIES COMPILE_FLAGS "${CMAKE_SHARED_LIBRARY_CXX_FLAGS}")
+set_target_properties(TgaImageConverterObjects PROPERTIES COMPILE_FLAGS "-DTgaImageConverterObjects_EXPORTS ${CMAKE_SHARED_LIBRARY_CXX_FLAGS}")
 
 add_plugin(TgaImageConverter ${MAGNUM_PLUGINS_IMAGECONVERTER_INSTALL_DIR}
     TgaImageConverter.conf

src/Plugins/TgaImageConverter/TgaImageConverter.h

@@ -31,7 +31,7 @@
 #include <Trade/AbstractImageConverter.h>
 
 #ifndef DOXYGEN_GENERATING_OUTPUT
-#ifdef TgaImageConverter_EXPORTS
+#if defined(TgaImageConverter_EXPORTS) || defined(TgaImageConverterObjects_EXPORTS)
     #define MAGNUM_TRADE_TGAIMAGECONVERTER_EXPORT CORRADE_VISIBILITY_EXPORT
 #else
     #define MAGNUM_TRADE_TGAIMAGECONVERTER_EXPORT CORRADE_VISIBILITY_IMPORT

src/Plugins/TgaImporter/CMakeLists.txt

@@ -30,7 +30,7 @@ set(TgaImporter_HEADERS
     TgaImporter.h)
 
 add_library(TgaImporterObjects OBJECT ${TgaImporter_SRCS})
-set_target_properties(TgaImporterObjects PROPERTIES COMPILE_FLAGS "${CMAKE_SHARED_LIBRARY_CXX_FLAGS}")
+set_target_properties(TgaImporterObjects PROPERTIES COMPILE_FLAGS "-DTgaImporterObjects_EXPORTS ${CMAKE_SHARED_LIBRARY_CXX_FLAGS}")
 
 add_plugin(TgaImporter ${MAGNUM_PLUGINS_IMPORTER_INSTALL_DIR}
     TgaImporter.conf

src/Plugins/TgaImporter/TgaImporter.h

@@ -32,7 +32,7 @@
 #include <Trade/AbstractImporter.h>
 
 #ifndef DOXYGEN_GENERATING_OUTPUT
-#ifdef TgaImporter_EXPORTS
+#if defined(TgaImporter_EXPORTS) || defined(TgaImporterObjects_EXPORTS)
     #define MAGNUM_TRADE_TGAIMPORTER_EXPORT CORRADE_VISIBILITY_EXPORT
 #else
     #define MAGNUM_TRADE_TGAIMPORTER_EXPORT CORRADE_VISIBILITY_IMPORT