CMake: avoid neeless variable expansion

CMake will automatically expand strings that are variable names in if().

CMakeLists.txt

@@ -51,9 +51,9 @@ if(CCACHE_FOUND)
     endif()
 endif(CCACHE_FOUND)
 
-if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
-    if(${RAPIDJSON_ENABLE_INSTRUMENTATION_OPT})
-        if("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "powerpc" OR "${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64" OR "${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64le")
+if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    if(RAPIDJSON_ENABLE_INSTRUMENTATION_OPT)
+        if(CMAKE_SYSTEM_PROCESSOR STREQUAL "powerpc" OR CMAKE_SYSTEM_PROCESSOR STREQUAL "ppc64" OR CMAKE_SYSTEM_PROCESSOR STREQUAL "ppc64le")
           set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -mcpu=native")
         else()
           #FIXME: x86 is -march=native, but doesn't mean every arch is this option. To keep original project's compatibility, I leave this except POWER.
@@ -84,7 +84,7 @@ if ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "GNU")
         endif()
     endif()
 elseif (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
-    if("${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "powerpc" OR "${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64" OR "${CMAKE_SYSTEM_PROCESSOR}" STREQUAL "ppc64le")
+    if(CMAKE_SYSTEM_PROCESSOR STREQUAL "powerpc" OR CMAKE_SYSTEM_PROCESSOR STREQUAL "ppc64" OR CMAKE_SYSTEM_PROCESSOR STREQUAL "ppc64le")
       set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -mcpu=native")
     else()
       #FIXME: x86 is -march=native, but doesn't mean every arch is this option. To keep original project's compatibility, I leave this except POWER.
@@ -105,7 +105,7 @@ elseif (CMAKE_CXX_COMPILER_ID MATCHES "Clang")
             set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fsanitize=undefined")
         endif()
     endif()
-elseif ("${CMAKE_CXX_COMPILER_ID}" STREQUAL "MSVC")
+elseif (CMAKE_CXX_COMPILER_ID STREQUAL "MSVC")
     add_definitions(-D_CRT_SECURE_NO_WARNINGS=1)
     set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} /EHsc")
 elseif (CMAKE_CXX_COMPILER_ID MATCHES "XL")