dllexport ASSIMP_API in all Windows compilers, not just MSVC

This fixes a problem mentioned in issue #2685 where the libassimp.dll
compiled with MinGW doesn't export any symbols.

include/assimp/defs.h

@@ -126,16 +126,14 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  * GENBOUNDINGBOXES */
 //////////////////////////////////////////////////////////////////////////
 
-#ifdef _MSC_VER
+#ifdef _WIN32
 #   undef ASSIMP_API
-
     //////////////////////////////////////////////////////////////////////////
     /* Define 'ASSIMP_BUILD_DLL_EXPORT' to build a DLL of the library */
     //////////////////////////////////////////////////////////////////////////
 #   ifdef ASSIMP_BUILD_DLL_EXPORT
 #       define ASSIMP_API __declspec(dllexport)
 #       define ASSIMP_API_WINONLY __declspec(dllexport)
-#       pragma warning (disable : 4251)
 
     //////////////////////////////////////////////////////////////////////////
     /* Define 'ASSIMP_DLL' before including Assimp to link to ASSIMP in
@@ -148,7 +146,19 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 #       define ASSIMP_API
 #       define ASSIMP_API_WINONLY
 #   endif
+#elif defined(SWIG)
+
+    /* Do nothing, the relevant defines are all in AssimpSwigPort.i */
+
+#else
+#   define ASSIMP_API __attribute__ ((visibility("default")))
+#   define ASSIMP_API_WINONLY
+#endif
 
+#ifdef _MSC_VER
+#   ifdef ASSIMP_BUILD_DLL_EXPORT
+#       pragma warning (disable : 4251)
+#   endif
     /* Force the compiler to inline a function, if possible
      */
 #   define AI_FORCE_INLINE __forceinline
@@ -156,17 +166,12 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     /* Tells the compiler that a function never returns. Used in code analysis
      * to skip dead paths (e.g. after an assertion evaluated to false). */
 #   define AI_WONT_RETURN __declspec(noreturn)
-
 #elif defined(SWIG)
 
     /* Do nothing, the relevant defines are all in AssimpSwigPort.i */
 
 #else
-
 #   define AI_WONT_RETURN
-
-#   define ASSIMP_API __attribute__ ((visibility("default")))
-#   define ASSIMP_API_WINONLY
 #   define AI_FORCE_INLINE inline
 #endif // (defined _MSC_VER)