Compatibility with Autoconf 2.70+

configure.ac

@@ -21,11 +21,26 @@ AM_INIT_AUTOMAKE([1.16.1 foreign])
 AM_MAINTAINER_MODE([enable])
 AM_EXTRA_RECURSIVE_TARGETS([examples])
 
-m4_pushdef([_AC_PROG_CC_C89],
-  [rm -rf conftest.dSYM # needed when configured on MacOS with CFLAGS='-g'
-   _AC_PROG_CC_C99([], [AC_MSG_FAILURE([C compiler does not support ISO C99])])])dnl
-AC_PROG_CC
-m4_popdef([_AC_PROG_CC_C89])dnl
+dnl Tests for the C compiler:
+dnl   Make sure conftest.dSYM is removed when configured on MacOS with
+dnl   CFLAGS='-g':
+m4_pushdef([_AM_PROG_CC_C_O], m4_bpatsubst(m4_dquote(m4_defn([_AM_PROG_CC_C_O])),
+  [rm -f\(.* conftest\*.*\)], [rm -rf\1]))dnl
+dnl   Check for ISO C99 support
+dnl   (Autoconf 2.69- and 2.70+ have different documented ways for this):
+m4_case(m4_version_compare(AC_AUTOCONF_VERSION,[2.70]),
+  [-1], [AC_PROG_CC_C99],
+  [0],  [dnl
+dnl   Fix a bug in Autoconf 2.70 (see https://savannah.gnu.org/support/?110396):
+         m4_pushdef([_AC_PROG_CC_C99],
+         m4_bpatsubst(m4_dquote(m4_defn([_AC_PROG_CC_C99])),
+           [\[ac_c_conftest_c89_program\]], [[ac_c_conftest_c99_program]]))dnl
+         AC_PROG_CC
+         m4_popdef([_AC_PROG_CC_C99])],
+  [1],  [AC_PROG_CC])
+AS_VAR_IF([ac_cv_prog_cc_c99], [no],
+  [AC_MSG_FAILURE([unable to detect C compiler flag needed to accept ISO C99])])
+m4_popdef([AC_PROG_CC_C_O])dnl
 ACX_PROG_CC_POSIX([2001])
 AC_C_RESTRICT