fix(cmake): fix set_if_higher (deepmodeling#3977)

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## Summary by CodeRabbit

- **Bug Fixes**
- Improved the `set_if_higher` macro to correctly interpret
`${VARIABLE}` as a variable name rather than a string, enhancing
variable comparison accuracy.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Signed-off-by: Jinzhe Zeng <jinzhe.zeng@rutgers.edu>

source/CMakeLists.txt

@@ -11,7 +11,8 @@ set(DEEPMD_C_ROOT
 
 set(CMAKE_CXX_STANDARD 11)
 macro(set_if_higher VARIABLE VALUE)
-  if("${VARIABLE}" LESS "${VALUE}")
+  # ${VARIABLE} is a variable name, not a string
+  if(${VARIABLE} LESS "${VALUE}")
     set(${VARIABLE} ${VALUE})
   endif()
 endmacro()