Add a configure test for C99-ish "for" declarations.

[dxcommon.git] / m4 / fordecl.m4

diff --git a/m4/fordecl.m4 b/m4/fordecl.m4
new file mode 100644 (file)
index 0000000..c36527d
--- /dev/null
+++ b/m4/fordecl.m4
@@ -0,0 +1,38 @@
+# Copyright © 2023 Nick Bowler
+#
+# License WTFPL2: Do What The Fuck You Want To Public License, version 2.
+# This is free software: you are free to do what the fuck you want to.
+# There is NO WARRANTY, to the extent permitted by law.
+
+# DX_C_FOR_DECLARATIONS
+#
+# Probe whether for-loop declarations are accepted by the C compiler,
+# as some pre-C99 implementations do have them.  If supported, the cache
+# variable dx_cv_have_for_decls is set to "yes" and the HAVE_FOR_DECLS
+# macro is defined to 1.
+#
+# We skip the test (and define the macro unconditionally) if Autoconf
+# has previously determined that the C compiler supports C99 or a newer
+# standard, since the C99 test program checks this.  However, autoconf-2.70
+# has a broken check which indicates C99 compatibility for C89 compilers,
+# so on this version we still do the check if C99 support is indicated.
+#
+# Annoyingly, autoconf-2.71 removed the assignment of ac_cv_prog_cc_c99,
+# even though this was actual documented behaviour...
+AC_DEFUN([DX_C_FOR_DECLARATIONS],
+[AS_CASE([${ac_cv_prog_cc_c99-no}/${ac_prog_cc_stdc-no}],
+  [m4_do([no/no|*/c89],
+    [_DX_IF_AC270_C99_BUG([|*/c99])])],
+  [AC_CACHE_CHECK([if $CC supports for-loop declarations],
+    [dx_cv_have_for_decls],
+    [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([], [for (int i; i = 1;) return i;])],
+    [dx_cv_have_for_decls=yes], [dx_cv_have_for_decls=no])])],
+  [dx_cv_have_for_decls=yes])
+AS_CASE([$dx_cv_have_for_decls],
+  [yes], [AC_DEFINE([HAVE_FOR_DECLS], [1],
+    [Define to 1 if the C compiler supports for-loop declarations.])])])
+
+m4_define([_DX_IF_AC270_C99_BUG], [m4_ifdef([_AC_PROG_CC_C99],
+  [m4_bmatch(m4_defn([_AC_PROG_CC_C99]),
+    [_AC_C_STD_TRY.\[c99\], *\[ac_c_conftest_c89_program\]],
+    [$1])])])