diff --git a/config.d/compiling.m4 b/config.d/compiling.m4
index 75f1fd13b..f9fa50942 100644
--- a/config.d/compiling.m4
+++ b/config.d/compiling.m4
@@ -56,7 +56,7 @@ if test "x$GCC" = xyes; then
 	shift
 	args="$*"
 	AC_MSG_CHECKING(for gcc version)
-	CCVER="gcc `$CC --version | grep '[[0-9]]\.[[0-9]]' | sed -e 's/.*(GCC)//' -e 's/[[^0-9]]*\([[0-9.]]*\).*/\1/'`"
+	CCVER="gcc `$CC --version | grep '[[0-9]]\.[[0-9]]' | sed -e 's/.*([[^)]]*)//' -e 's/[[^0-9]]*\([[0-9.]]*\).*/\1/'`"
 	set $CCVER
 	save_IFS="$IFS"
 	IFS="."
@@ -341,7 +341,7 @@ if test "x$leave_cflags_alone" != xyes; then
 			[[bool flag = true;]],
 			[[return flag ? 1 : 0]])],
 		[],
-		[QF_CC_OPTION_TEST(-std=gnu23,[],QF_CC_OPTION(-std=gnu2x))]
+		[QF_CC_OPTION_TEST(-std=gnu23,QF_CC_OPTION(-std=gnu23),QF_CC_OPTION(-std=gnu2x))]
 	)
 fi
 AC_MSG_CHECKING([for c23])
diff --git a/config.d/header_files.m4 b/config.d/header_files.m4
index 93513dc8c..c8133457d 100644
--- a/config.d/header_files.m4
+++ b/config.d/header_files.m4
@@ -37,16 +37,16 @@ AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include "fnmatch.h"]], [[int (*foo)() = fn
 ])
 
 AC_MSG_CHECKING(for strnlen in string.h)
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include "string.h"]], [[int (*foo)() = strnlen;]])],[AC_DEFINE(HAVE_STRNLEN_PROTO, 1, Define this if strnlen is prototyped in string.h)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include "string.h"]], [[size_t (*foo)(const char *, size_t) = strnlen;]])],[AC_DEFINE(HAVE_STRNLEN_PROTO, 1, Define this if strnlen is prototyped in string.h)
 	AC_MSG_RESULT(yes)],[AC_MSG_RESULT(no)
 ])
 
 AC_MSG_CHECKING(for strndup in string.h)
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include "string.h"]], [[int (*foo)() = strndup;]])],[AC_DEFINE(HAVE_STRNDUP_PROTO, 1, Define this if strndup is prototyped in string.h)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include "string.h"]], [[char *(*foo)(const char *, size_t) = strndup;]])],[AC_DEFINE(HAVE_STRNDUP_PROTO, 1, Define this if strndup is prototyped in string.h)
 	AC_MSG_RESULT(yes)],[AC_MSG_RESULT(no)
 ])
 
 AC_MSG_CHECKING(for strcasestr in string.h)
-AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include "string.h"]], [[int (*foo)() = strcasestr;]])],[AC_DEFINE(HAVE_STRCASESTR_PROTO, 1, Define this if strcasestr is prototyped in string.h)
+AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[#include "string.h"]], [[char *(*foo)(const char *, const char *) = strcasestr;]])],[AC_DEFINE(HAVE_STRCASESTR_PROTO, 1, Define this if strcasestr is prototyped in string.h)
 	AC_MSG_RESULT(yes)],[AC_MSG_RESULT(no)
 ])
diff --git a/include/compat.h b/include/compat.h
index 07c871667..f1d608a50 100644
--- a/include/compat.h
+++ b/include/compat.h
@@ -96,7 +96,7 @@ size_t strnlen (const char *str, size_t len);
 #endif
 #ifdef HAVE_STRNDUP
 # ifndef HAVE_STRNDUP_PROTO
-size_t strndup (const char *str, size_t len);
+char *strndup (const char *str, size_t len);
 # endif
 #else
 # define strndup Q_strndup