Fix building unit tests targetting windows

Fixes MSVC's LNK4217 warnings and MinGW errors.

src/CMakeLists.txt

@@ -116,12 +116,15 @@ TARGET_INCLUDE_DIRECTORIES(lmms
 	PUBLIC ${CMAKE_CURRENT_BINARY_DIR}
 )
 
-# GENERATE_EXPORT_HEADER doesn't do this automatically for OBJECT libraries
+# CMake doesn't define target_EXPORTS for OBJECT libraries.
+# See the documentation of DEFINE_SYMBOL for details.
+# Also add LMMS_STATIC_DEFINE for targets linking against it.
 TARGET_COMPILE_DEFINITIONS(lmmsobjs
 	PRIVATE -Dlmmsobjs_EXPORTS
+	INTERFACE -DLMMS_STATIC_DEFINE
 )
 TARGET_COMPILE_DEFINITIONS(lmms
-	PRIVATE -Dlmmsobjs_EXPORTS
+	PRIVATE $<TARGET_PROPERTY:lmmsobjs,INTERFACE_COMPILE_DEFINITIONS>
 )
 
 # Set Visual Studio startup project to lmms
@@ -389,4 +392,4 @@ ELSE(NOT MSVC)
 		#				"${VCPKG_ROOT}/bin/libsoundio.dll"
 		#			DESTINATION .)
 		#ENDIF()
-ENDIF(NOT MSVC)
+ENDIF(NOT MSVC)