[PSEH] Add implementation for GCC amd64

Also, put include directory next to the library and use
target_include_directories(.. INTERFACE ..) to get this right.
This is because :
 - Having includes & implementation in two different places buggers me
 - This makes sure that there is no "if it compiles everything is fine" behaviour from anyone
   because now even static libraries need it for GCC amd64 build
Also add __USE_PSEH2__ define for the non SEH-aware compilers out there and use it in a few headers
where we define macros involving __try

sdk/include/psdk/rpc.h

@@ -5,7 +5,7 @@
 #endif /* _INC_WINDOWS */
 #endif
 
-#ifndef RC_INVOKED
+#if defined(__USE_PSEH2__) && !defined(RC_INVOKED)
 #include  <pseh/pseh2.h>
 #endif
 
@@ -122,7 +122,7 @@ typedef int RPC_STATUS;
     #include <excpt.h>
     #include <winerror.h>
 
-    #ifndef __GNUC__
+    #ifndef __USE_PSEH2__
         #define RpcTryExcept __try {
         #define RpcExcept(expr) } __except (expr) {
         #define RpcEndExcept }