build: don't check for ansidecl.h

This is an internal GCC header that serves no purpose being checked for.

The same header has already been removed from libxml2: https://gitlab.gnome.org/GNOME/libxml2/commit/ff628d46a1301d76e0217c23c7ef53c5f1faed45.

configure.ac

@@ -168,7 +168,7 @@
 dnl Math detection
 dnl
 
-AC_CHECK_HEADERS(ieeefp.h nan.h math.h fp_class.h float.h ansidecl.h)
+AC_CHECK_HEADERS(ieeefp.h nan.h math.h fp_class.h float.h)
 AC_CHECK_HEADERS(sys/timeb.h time.h sys/stat.h sys/select.h stdarg.h)
 AC_CHECK_HEADERS(errno.h)
 AC_CHECK_FUNCS(stat _stat)