Setup System_Stats extention on Windows

Hello, 
I'm new to Postgres and pgAdmin. 

I am trying to build the "Postgres System Stats extension". First, I've set PG_INCLUDE_DIR and PG_LIB_DIR environment variables as follows: 
                 PG_INCLUDE_DIR=C:\Program Files\PostgreSQL\15\include
                 PG_LIB_DIR=C:\Program Files\PostgreSQL\15\lib

 But when I build it on Windows cmd using the command: msbuild system_stats.vcxproj /p:configuration=debug,
 it gives me the following error: 'fatal error C1083: Cannot open include file: 'postgres.h': No such file or directory'.
 I've found that file: 'postgres.h' exists already on my path: C:\Program Files\PostgreSQL\15\include\server.

Thanks, 
Mohamed