fix cmake build when ZSTD_BUILD_TESTS is disabled

Author: Cyan4973

build/cmake/CMakeModules/AddZstdCompilationFlags.cmake

@@ -15,18 +15,35 @@ function(EnableCompilerFlag _flag _C _CXX _LD)
     string(REGEX REPLACE "[^A-Za-z0-9]+" "_" varname "${varname}")
     string(REGEX REPLACE "^_+" "" varname "${varname}")
     string(TOUPPER "${varname}" varname)
+
+    # _C is expected to be a direct boolean-like value (e.g., "ON", "OFF", true, false)
+    # passed from ADD_ZSTD_COMPILATION_FLAGS, indicating if the C flag should be checked.
     if (_C)
         CHECK_C_COMPILER_FLAG(${_flag} C_FLAG_${varname})
         if (C_FLAG_${varname})
             set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${_flag}" PARENT_SCOPE)
         endif ()
     endif ()
-    if (_CXX)
+
+    # _CXX is unique: it can be a direct boolean-like value (e.g., false) or
+    # the *name* of a variable (e.g., "ZSTD_ENABLE_CXX") that holds the boolean-like value.
+    # Therefore, it must be dereferenced with ${} to get its actual value.
+    if (${_CXX}) # Dereference _CXX to get the value of the variable whose name is passed
         CHECK_CXX_COMPILER_FLAG(${_flag} CXX_FLAG_${varname})
         if (CXX_FLAG_${varname})
             set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${_flag}" PARENT_SCOPE)
         endif ()
+        # If CHECK_CXX_COMPILER_FLAG was run, CXX_FLAG_${varname} is now defined (as TRUE or FALSE).
+    else()
+        # If the condition ${_CXX} is false (e.g., ZSTD_ENABLE_CXX is OFF),
+        # CHECK_CXX_COMPILER_FLAG is skipped.
+        # We must ensure CXX_FLAG_${varname} is defined as FALSE for subsequent logic
+        # that might read it (e.g., for ZSTD_HAS_NOEXECSTACK).
+        set(CXX_FLAG_${varname} FALSE PARENT_SCOPE)
     endif ()
+
+    # _LD is expected to be a direct boolean-like value (e.g., "ON", "OFF", true, false)
+    # passed from ADD_ZSTD_COMPILATION_FLAGS, indicating if the Linker flag should be checked.
     if (_LD)
         # We never add a linker flag with CMake < 3.18. We will
         # implement CHECK_LINKER_FLAG() like feature for CMake < 3.18