[cpp.predefined,version.syn] Add index entries for "feature-test macro" (#6564)

Author: Quuxplusone

source/preprocessor.tex

@@ -1794,6 +1794,8 @@
 an \impldef{text of \mname{TIME} when time of translation is not available} valid time shall be supplied.
 \end{description}
 
+\indextext{macro!feature-test}%
+\indextext{feature-test macro|see{macro, feature-test}}%
 \begin{LongTable}{Feature-test macros}{cpp.predefined.ft}{ll}
 \\ \topline
 \lhdr{Macro name} & \rhdr{Value} \\ \capsep

source/support.tex

@@ -541,6 +541,7 @@
 (e.g., version number and release date).
 
 \pnum
+\indextext{macro!feature-test}%
 Each of the macros defined in \libheader{version} is also defined
 after inclusion of any member of the set of library headers
 indicated in the corresponding comment in this synopsis.