https://src.fedoraproject.org/rpms/libmcrypt/blob/rawhide/f/libmcrypt-c99.patch

Add return and argument types to fake prototypes in mcrypt_symb.c.
This avoids build failures with future compilers that do not support
implicit function declarations.

--- a/lib/Makefile.am
+++ b/lib/Makefile.am
@@ -23,8 +23,8 @@ mcrypt_symb.c: mcrypt_internal.h
 	@echo "/* This is automatically created. Don't touch... */" >> mcrypt_symb.c
 	@echo "" >> mcrypt_symb.c
 	-@for i in $(EXTRA_ALGOS); do \
-		if test -f ../modules/algorithms/$$i.c; then cat ../modules/algorithms/$$i.c 2>/dev/null|grep define|grep LTX|awk '{print "extern "$$3"();";}' >> mcrypt_symb.c 2>/dev/null; fi; \
-		if test -f ../modules/modes/$$i.c; then cat ../modules/modes/$$i.c 2>/dev/null|grep define|grep LTX|awk '{print "extern "$$3"();";}' >> mcrypt_symb.c 2>/dev/null; fi; \
+		if test -f ../modules/algorithms/$$i.c; then cat ../modules/algorithms/$$i.c 2>/dev/null|grep define|grep LTX|awk '{print "extern void "$$3"(void);";}' >> mcrypt_symb.c 2>/dev/null; fi; \
+		if test -f ../modules/modes/$$i.c; then cat ../modules/modes/$$i.c 2>/dev/null|grep define|grep LTX|awk '{print "extern void "$$3"(void);";}' >> mcrypt_symb.c 2>/dev/null; fi; \
 	done
 	@echo "" >> mcrypt_symb.c
 	@echo "const mcrypt_preloaded mps[] = {" >> mcrypt_symb.c
--- a/lib/Makefile.in
+++ b/lib/Makefile.in
@@ -561,8 +561,8 @@ mcrypt_symb.c: mcrypt_internal.h
 	@echo "/* This is automatically created. Don't touch... */" >> mcrypt_symb.c
 	@echo "" >> mcrypt_symb.c
 	-@for i in $(EXTRA_ALGOS); do \
-		if test -f ../modules/algorithms/$$i.c; then cat ../modules/algorithms/$$i.c 2>/dev/null|grep define|grep LTX|awk '{print "extern "$$3"();";}' >> mcrypt_symb.c 2>/dev/null; fi; \
-		if test -f ../modules/modes/$$i.c; then cat ../modules/modes/$$i.c 2>/dev/null|grep define|grep LTX|awk '{print "extern "$$3"();";}' >> mcrypt_symb.c 2>/dev/null; fi; \
+		if test -f ../modules/algorithms/$$i.c; then cat ../modules/algorithms/$$i.c 2>/dev/null|grep define|grep LTX|awk '{print "extern void "$$3"(void);";}' >> mcrypt_symb.c 2>/dev/null; fi; \
+		if test -f ../modules/modes/$$i.c; then cat ../modules/modes/$$i.c 2>/dev/null|grep define|grep LTX|awk '{print "extern void "$$3"(void);";}' >> mcrypt_symb.c 2>/dev/null; fi; \
 	done
 	@echo "" >> mcrypt_symb.c
 	@echo "const mcrypt_preloaded mps[] = {" >> mcrypt_symb.c

