QAR-1619: Fix Windows link errors when building INTEGRATION_TEST_lllazy.

The problem arose because we were setting LL_COMMON_BUILD in
llcommon/CMakeLists.txt, not only for the library build itself but also for
its LL_ADD_INTEGRATION_TEST tests. This told all the headers compiled into the
INTEGRATION_TEST_lllazy executable that the executable was providing all the
llcommon symbols, rather than importing them.
The solution is to switch to the llcommon_EXPORTS symbol automagically defined
by CMake when building the llcommon shared library itself.

indra/llcommon/CMakeLists.txt

@@ -242,7 +242,6 @@ list(APPEND llcommon_SOURCE_FILES ${llcommon_HEADER_FILES})
 
 if(LLCOMMON_LINK_SHARED)
     add_library (llcommon SHARED ${llcommon_SOURCE_FILES})
-    add_definitions(-DLL_COMMON_BUILD=1)
 
     if(SHARED_LIB_STAGING_DIR)
         # *FIX:Mani ---
@@ -290,7 +289,6 @@ target_link_libraries(
 
 add_dependencies(llcommon stage_third_party_libs)
 
-
 include(LLAddBuildTest)
 SET(llcommon_TEST_SOURCE_FILES
   )

indra/llcommon/llpreprocessor.h

@@ -149,11 +149,16 @@
 #endif // LL_WINDOWS
 
 #if LL_COMMON_LINK_SHARED
-# if LL_COMMON_BUILD
+// CMake automagically defines llcommon_EXPORTS only when building llcommon
+// sources, and only when llcommon is a shared library (i.e. when
+// LL_COMMON_LINK_SHARED). We must still test LL_COMMON_LINK_SHARED because
+// otherwise we can't distinguish between (non-llcommon source) and (llcommon
+// not shared).
+# if defined(llcommon_EXPORTS)
 #   define LL_COMMON_API LL_DLLEXPORT
-# else //LL_COMMON_BUILD
+# else //llcommon_EXPORTS
 #   define LL_COMMON_API LL_DLLIMPORT
-# endif //LL_COMMON_BUILD
+# endif //llcommon_EXPORTS
 #else // LL_COMMON_LINK_SHARED
 # define LL_COMMON_API
 #endif // LL_COMMON_LINK_SHARED