build: make the C99 requirement explicit (#193)

Recent code uses C99 syntax (as reported in #163, and therefore it
will fail to build unless C99 (or higher) support is required by the
compiler (gcc >= 5 does it by default).

For those unfortunate to need an older compiler that does require a
flag to enable that, make sure that both autoconf and cmake make it
explicit.

While testing the change with the ancient autoconf version listed as
a prerequisite, noticed that the LT_INIT syntax introduced with 91485e5
(Update configure.ac for latest version of autoconf., 2021-02-02), was
not supported, so update the autoconf minimum as well.

CMakeLists.txt

@@ -100,11 +100,14 @@
 # 2021-08-28 PH increased minimum version
 # 2021-08-28 PH added test for realpath()
 # 2022-12-10 PH added support for pcre2posix_test
+# 2023-01-15 Carlo added C99 as the minimum required
 
 # Increased minimum to 2.8.5 to support GNUInstallDirs.
 # Increased minimum to 3.1 to support imported targets.
 CMAKE_MINIMUM_REQUIRED(VERSION 3.1)
 PROJECT(PCRE2 C)
+set(CMAKE_C_STANDARD 99)
+set(CMAKE_C_STANDARD_REQUIRED TRUE)
 
 # Set policy CMP0026 to avoid warnings for the use of LOCATION in
 # GET_TARGET_PROPERTY. This should no longer be required.

configure.ac

@@ -22,7 +22,7 @@ m4_define(libpcre2_posix_version, [3:4:0])
 # NOTE: The CMakeLists.txt file searches for the above variables in the first
 # 50 lines of this file. Please update that if the variables above are moved.
 
-AC_PREREQ([2.60])
+AC_PREREQ([2.62])
 AC_INIT([PCRE2],pcre2_major.pcre2_minor[]pcre2_prerelease,[],[pcre2])
 AC_CONFIG_SRCDIR([src/pcre2.h.in])
 AM_INIT_AUTOMAKE([dist-bzip2 dist-zip])
@@ -42,7 +42,7 @@ AC_CONFIG_MACRO_DIR([m4])
 
 remember_set_CFLAGS="$CFLAGS"
 
-AC_PROG_CC
+m4_version_prereq(2.70, [AC_PROG_CC], [AC_PROG_CC_C99])
 AM_PROG_CC_C_O
 AC_USE_SYSTEM_EXTENSIONS