Suppress H2O compilation warnings (#15378)

* Suppress H2O compilation warnings

* Fix typo

* Update flags

* Another fix typo

* Flags update once more

* Remove duplicate flag

* Move flag

Makefile.am

@@ -1059,7 +1059,7 @@ if ENABLE_H2O
 noinst_LIBRARIES += libh2o.a
 
 # until h2o updates support for OpenSSL 3.0 we silence the warnings
-libh2o_a_CFLAGS = $(CFLAGS) -Wno-deprecated-declarations -Wno-unused-parameter -Wno-sign-compare -Wno-missing-field-initializers -DH2O_USE_LIBUV=0 $(libh2o_a_INCLUDES)
+libh2o_a_CFLAGS = $(CFLAGS) -Wno-old-style-declaration -Wno-deprecated-declarations -Wno-unused-parameter -Wno-sign-compare -Wno-missing-field-initializers -DH2O_USE_LIBUV=0 $(libh2o_a_INCLUDES)
 
 if LINUX
     libh2o_a_CFLAGS += -D_GNU_SOURCE

configure.ac

@@ -908,7 +908,7 @@ if test "${enable_h2o}" != "no"; then
 
     if test "${can_build_h2o}" = "yes"; then
         AC_DEFINE([ENABLE_H2O], [1], [H2O web server])
-        H2O_CFLAGS="-I\$(abs_top_srcdir)/web/server/h2o/libh2o/include -I\$(abs_top_srcdir)/web/server/h2o/libh2o/deps/picotls/include -I\$(abs_top_srcdir)/web/server/h2o/libh2o/deps/quicly/include -DH2O_USE_LIBUV=0"
+        H2O_CFLAGS="-Wno-unused-result -Wno-misleading-indentation -Wno-format-security -Wno-format -Wno-unused-function -Wno-unused-parameter -Wno-type-limits -Wno-unused-but-set-variable -Wno-implicit-fallthrough -I\$(abs_top_srcdir)/web/server/h2o/libh2o/include -I\$(abs_top_srcdir)/web/server/h2o/libh2o/deps/picotls/include -I\$(abs_top_srcdir)/web/server/h2o/libh2o/deps/quicly/include -DH2O_USE_LIBUV=0"
     fi
 fi
 AM_CONDITIONAL([ENABLE_H2O], [test "${can_build_h2o}" = "yes"])