diff -uNr groff-1.22.4/lib/math.in.h groff-1.22.4.mod/lib/math.in.h
--- groff-1.22.4/lib/math.in.h	2018-12-23 17:34:15.000000000 +0300
+++ groff-1.22.4.mod/lib/math.in.h	2021-09-30 15:36:00.887961530 +0300
@@ -33,6 +33,7 @@
 # include <fp.h>
 #endif
 
+#include <config.h>
 #ifndef _GL_INLINE_HEADER_BEGIN
  #error "Please include config.h first."
 #endif
