[cfenv.syn] Add "optional" markers for macros from the C library that are optional (as defined by C)

Author: tkoeppe

source/numerics.tex

@@ -179,16 +179,16 @@
 \indexlibrary{\idxcode{FE_DFL_ENV}}%
 \begin{codeblock}
 #define FE_ALL_EXCEPT @\seebelow@
-#define FE_DIVBYZERO @\seebelow@
-#define FE_INEXACT @\seebelow@
-#define FE_INVALID @\seebelow@
-#define FE_OVERFLOW @\seebelow@
-#define FE_UNDERFLOW @\seebelow@
-
-#define FE_DOWNWARD @\seebelow@
-#define FE_TONEAREST @\seebelow@
-#define FE_TOWARDZERO @\seebelow@
-#define FE_UPWARD @\seebelow@
+#define FE_DIVBYZERO @\seebelow@    // optional
+#define FE_INEXACT @\seebelow@      // optional
+#define FE_INVALID @\seebelow@      // optional
+#define FE_OVERFLOW @\seebelow@     // optional
+#define FE_UNDERFLOW @\seebelow@    // optional
+
+#define FE_DOWNWARD @\seebelow@     // optional
+#define FE_TONEAREST @\seebelow@    // optional
+#define FE_TOWARDZERO @\seebelow@   // optional
+#define FE_UPWARD @\seebelow@       // optional
 
 #define FE_DFL_ENV @\seebelow@