build: Set -DUSE_IMPORT_EXPORT in pkgconfig files on Windows

MSVC consumers of the shared library need to define this macro when
linking to aws-cpp-sdk-* libs, or they will get undefined reference
errors. Also doesn't hurt to define this with MinGW, so add it
unconditionally on Windows.

Related to amzn/amazon-s3-gst-plugin#13

Author: nirbheek

CMakeLists.txt

@@ -266,6 +266,13 @@ else()
     set(PKG_CONFIG_LIB_DIRECTORY "${LIBRARY_DIRECTORY}")
 endif()
 
+# MSVC consumers of the shared library need to define this macro when linking
+# to aws-cpp-sdk-* libs, or they will get undefined reference errors. Also
+# doesn't hurt to define this with MinGW, so add it unconditionally on Windows.
+if(WIN32 AND BUILD_SHARED_LIBS)
+    set(PKG_CONFIG_EXPORT_CFLAGS "-DUSE_IMPORT_EXPORT")
+endif()
+
 if (ENABLE_ADDRESS_SANITIZER)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=address -g -fno-omit-frame-pointer")
     if (CMAKE_COMPILER_IS_GNUCXX AND CMAKE_CXX_COMPILER_VERSION VERSION_GREATER 7.1)

toolchains/pkg-config.pc.in

@@ -4,7 +4,7 @@ libdir=@CMAKE_INSTALL_PREFIX@/@PKG_CONFIG_LIB_DIRECTORY@
 Name: @PROJECT_NAME@
 Description: @PROJECT_DESCRIPTION@
 Version: @PROJECT_VERSION@
-Cflags: -I${includedir} @PKG_CONFIG_CFLAGS@
+Cflags: -I${includedir} @PKG_CONFIG_CFLAGS@ @PKG_CONFIG_EXPORT_CFLAGS@
 Libs: -L${libdir} -l@PROJECT_NAME@ 
 Libs.private: @ALL_DEP_LIBS_LINK_FLAGS@
 Requires: @PROJECT_LIBS_STRING@