Define NGHTTP2_EXTERN to __declspec(dllimport) when using nghttp2

The private global variable nghttp2_enable_strict_preface is also
marked as NGHTTP2_EXTERN, but it is test purpose only (test with
.dll), and not part of public API.  It could be removed in the future
release.

lib/Makefile.am

@@ -25,7 +25,8 @@ SUBDIRS = includes
 EXTRA_DIST = Makefile.msvc
 
 AM_CFLAGS = $(WARNCFLAGS)
-AM_CPPFLAGS = -I$(srcdir)/includes -I$(builddir)/includes @DEFS@
+AM_CPPFLAGS = -I$(srcdir)/includes -I$(builddir)/includes -DBUILDING_NGHTTP2 \
+	@DEFS@
 
 pkgconfigdir = $(libdir)/pkgconfig
 pkgconfig_DATA = libnghttp2.pc

lib/includes/nghttp2/nghttp2.h

@@ -51,7 +51,11 @@ extern "C" {
 #ifdef NGHTTP2_STATICLIB
 #define NGHTTP2_EXTERN
 #elif defined(WIN32)
+#ifdef BUILDING_NGHTTP2
 #define NGHTTP2_EXTERN __declspec(dllexport)
+#else /* !BUILDING_NGHTTP2 */
+#define NGHTTP2_EXTERN __declspec(dllimport)
+#endif /* !BUILDING_NGHTTP2 */
 #else  /* !defined(WIN32) */
 #define NGHTTP2_EXTERN
 #endif /* !defined(WIN32) */

lib/nghttp2_session.c

@@ -307,7 +307,8 @@ static void active_outbound_item_reset(nghttp2_active_outbound_item *aob,
 
 /* The global variable for tests where we want to disable strict
    preface handling. */
-int nghttp2_enable_strict_preface = 1;
+/* Specify NGHTTP2_EXTERN, so that we can test using Win build dll. */
+NGHTTP2_EXTERN int nghttp2_enable_strict_preface = 1;
 
 static int session_new(nghttp2_session **session_ptr,
                        const nghttp2_session_callbacks *callbacks,