Windows: Define DLL_EXPORT when building liblzma.dll with MSVC.

src/liblzma/common/common.h uses it to set __declspec(dllexport)
for the API symbols.

Thanks to Adam Walling.
diff --git a/windows/liblzma_dll.vcxproj b/windows/liblzma_dll.vcxproj
index 92e9838..d09f5e4 100644
--- a/windows/liblzma_dll.vcxproj
+++ b/windows/liblzma_dll.vcxproj
@@ -123,7 +123,7 @@
   </PropertyGroup>
   <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|Win32'">
     <ClCompile>
-      <PreprocessorDefinitions>WIN32;HAVE_CONFIG_H;_DEBUG;_WINDOWS;_USRDLL;LIBLZMADLL_EXPORTS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>WIN32;HAVE_CONFIG_H;DLL_EXPORT;_DEBUG;_WINDOWS;_USRDLL;LIBLZMADLL_EXPORTS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <RuntimeLibrary>MultiThreadedDebugDLL</RuntimeLibrary>
       <WarningLevel>Level3</WarningLevel>
       <DebugInformationFormat>ProgramDatabase</DebugInformationFormat>
@@ -141,7 +141,7 @@
   </ItemDefinitionGroup>
   <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Debug|x64'">
     <ClCompile>
-      <PreprocessorDefinitions>WIN32;HAVE_CONFIG_H;_DEBUG;_WINDOWS;_USRDLL;LIBLZMADLL_EXPORTS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>WIN32;HAVE_CONFIG_H;DLL_EXPORT;_DEBUG;_WINDOWS;_USRDLL;LIBLZMADLL_EXPORTS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <RuntimeLibrary>MultiThreadedDebugDLL</RuntimeLibrary>
       <WarningLevel>Level3</WarningLevel>
       <DebugInformationFormat>ProgramDatabase</DebugInformationFormat>
@@ -158,7 +158,7 @@
   </ItemDefinitionGroup>
   <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|Win32'">
     <ClCompile>
-      <PreprocessorDefinitions>WIN32;HAVE_CONFIG_H;NDEBUG;_WINDOWS;_USRDLL;LIBLZMADLL_EXPORTS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>WIN32;HAVE_CONFIG_H;DLL_EXPORT;NDEBUG;_WINDOWS;_USRDLL;LIBLZMADLL_EXPORTS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <RuntimeLibrary>MultiThreadedDLL</RuntimeLibrary>
       <WarningLevel>Level3</WarningLevel>
       <DebugInformationFormat>ProgramDatabase</DebugInformationFormat>
@@ -177,7 +177,7 @@
   </ItemDefinitionGroup>
   <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='Release|x64'">
     <ClCompile>
-      <PreprocessorDefinitions>WIN32;HAVE_CONFIG_H;NDEBUG;_WINDOWS;_USRDLL;LIBLZMADLL_EXPORTS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>WIN32;HAVE_CONFIG_H;DLL_EXPORT;NDEBUG;_WINDOWS;_USRDLL;LIBLZMADLL_EXPORTS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <RuntimeLibrary>MultiThreadedDLL</RuntimeLibrary>
       <WarningLevel>Level3</WarningLevel>
       <DebugInformationFormat>ProgramDatabase</DebugInformationFormat>
@@ -195,7 +195,7 @@
   </ItemDefinitionGroup>
   <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='ReleaseMT|Win32'">
     <ClCompile>
-      <PreprocessorDefinitions>WIN32;HAVE_CONFIG_H;NDEBUG;_WINDOWS;_USRDLL;LIBLZMADLL_EXPORTS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>WIN32;HAVE_CONFIG_H;DLL_EXPORT;NDEBUG;_WINDOWS;_USRDLL;LIBLZMADLL_EXPORTS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <RuntimeLibrary>MultiThreaded</RuntimeLibrary>
       <WarningLevel>Level3</WarningLevel>
       <DebugInformationFormat>ProgramDatabase</DebugInformationFormat>
@@ -214,7 +214,7 @@
   </ItemDefinitionGroup>
   <ItemDefinitionGroup Condition="'$(Configuration)|$(Platform)'=='ReleaseMT|x64'">
     <ClCompile>
-      <PreprocessorDefinitions>WIN32;HAVE_CONFIG_H;NDEBUG;_WINDOWS;_USRDLL;LIBLZMADLL_EXPORTS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
+      <PreprocessorDefinitions>WIN32;HAVE_CONFIG_H;DLL_EXPORT;NDEBUG;_WINDOWS;_USRDLL;LIBLZMADLL_EXPORTS;%(PreprocessorDefinitions)</PreprocessorDefinitions>
       <RuntimeLibrary>MultiThreaded</RuntimeLibrary>
       <WarningLevel>Level3</WarningLevel>
       <DebugInformationFormat>ProgramDatabase</DebugInformationFormat>