add CMake flag for building only the scalar target

PiperOrigin-RevId: 375090747
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 30ca357..4eb59af 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -41,6 +41,7 @@
 endif()
 
 set(HWY_CMAKE_ARM7 OFF CACHE BOOL "Set copts for ARMv7 with NEON?")
+set(HWY_CMAKE_SCALAR OFF CACHE BOOL "Only build scalar target")
 
 include(CheckCXXSourceCompiles)
 check_cxx_source_compiles(
@@ -164,6 +165,10 @@
 
 endif()  # !MSVC
 
+if (HWY_CMAKE_SCALAR)
+  list(APPEND HWY_FLAGS "-DHWY_DISABLED_TARGETS=~HWY_SCALAR")
+endif()
+
 add_library(hwy STATIC ${HWY_SOURCES})
 target_compile_options(hwy PRIVATE ${HWY_FLAGS})
 set_property(TARGET hwy PROPERTY POSITION_INDEPENDENT_CODE ON)