From: Daniel Salzman <daniel.salzman@nic.cz>
Date: Sun, 20 Feb 2022 20:38:35 +0100
Subject: [PATCH] Revert "configure: upgrade from AC_PROG_CC_C99 to AC_PROG_CC"

diff --git a/configure.ac b/configure.ac
index 6506197ed..c7df7f815 100644
--- a/configure.ac
+++ b/configure.ac
@@ -38,7 +38,8 @@ release_date=$($SED -n 's/^Knot DNS .* (\(.*\))/\1/p;q;' ${srcdir}/NEWS)
 AC_SUBST([RELEASE_DATE], $release_date)
 
 # Set compiler compatibility flags
-AC_PROG_CC
+AC_PROG_CC_C99 # AC_PROG_CC not supported by CentOS 7
+AM_PROG_CC_C_O # Needed by CentOS 7
 AC_PROG_CPP_WERROR
 
 # Set default CFLAGS