EnableCompilerFlag: Support multiple space-separated flags

This allows correct detection of linker noexecstack support.

When testing for support of multiple flags, the flags must be a list of
values separated by semicolons, not a string of values separated by
spaces. See https://gitlab.kitware.com/cmake/cmake/-/issues/26024

Enclose the flaglist variable in quotation marks when calling
CHECK_*_FLAG so that it is passed through unmodified.

After it is determined that a flag is supported, it is still the string
version of the flag variable that is appended to the corresponding
CMAKE_*_FLAGS variable.

Closes facebook#4056

build/cmake/CMakeModules/AddZstdCompilationFlags.cmake

@@ -12,18 +12,19 @@ if (ZSTD_HAVE_CHECK_LINKER_FLAG)
 endif()
 
 function(EnableCompilerFlag _flag _C _CXX _LD)
+    string(REGEX REPLACE " " ";" flaglist "${_flag}")
     string(REGEX REPLACE "\\+" "PLUS" varname "${_flag}")
     string(REGEX REPLACE "[^A-Za-z0-9]+" "_" varname "${varname}")
     string(REGEX REPLACE "^_+" "" varname "${varname}")
     string(TOUPPER "${varname}" varname)
     if (_C)
-        CHECK_C_COMPILER_FLAG(${_flag} C_FLAG_${varname})
+        CHECK_C_COMPILER_FLAG("${flaglist}" C_FLAG_${varname})
         if (C_FLAG_${varname})
             set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} ${_flag}" PARENT_SCOPE)
         endif ()
     endif ()
     if (_CXX)
-        CHECK_CXX_COMPILER_FLAG(${_flag} CXX_FLAG_${varname})
+        CHECK_CXX_COMPILER_FLAG("${flaglist}" CXX_FLAG_${varname})
         if (CXX_FLAG_${varname})
             set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} ${_flag}" PARENT_SCOPE)
         endif ()
@@ -39,7 +40,7 @@ function(EnableCompilerFlag _flag _C _CXX _LD)
         # results for this configuration,
         # see: https://gitlab.kitware.com/cmake/cmake/-/issues/22023
         if (ZSTD_HAVE_CHECK_LINKER_FLAG AND NOT MSVC)
-            CHECK_LINKER_FLAG(C ${_flag} LD_FLAG_${varname})
+            CHECK_LINKER_FLAG(C "${flaglist}" LD_FLAG_${varname})
         else ()
             set(LD_FLAG_${varname} false)
         endif ()