Move C4251 suppression entirely into the header

The macro function should be populated in the header itself, because not
only are there more compilers on Windows than just MSVC, but this header
gets installed, so the check needs to happen in the consuming code.

The check for __ICL is taken from the Hedley library, which makes this
suppression exclude the Intel compiler on Windows. I don't have the
Intel compiler to check whether this is correct.

Author: friendlyanon

cmake-init/templates/common/cmake/variables.cmake

@@ -12,10 +12,14 @@ endif()
 {if suppress}# ---- Suppress C4251 on Windows ----
 
 # Please see include/%(name)s/%(name)s.hpp for more details
-set(pragma_suppress_c4251 "#define %(uc_name)s_SUPPRESS_C4251")
-if(MSVC)
-  string(APPEND pragma_suppress_c4251 [[ _Pragma("warning(suppress:4251)")]])
-endif()
+set(pragma_suppress_c4251 "
+/* This needs to suppress only for MSVC */
+#if defined(_MSC_VER) && !defined(__ICL)
+#  define %(uc_name)s_SUPPRESS_C4251 _Pragma(\"warning(suppress:4251)\")
+#else
+#  define %(uc_name)s_SUPPRESS_C4251
+#endif
+")
 
 {end}# ---- Warning guard ----