Move iconv const check into autoconf

[NattyNarwhal]

Some systems (older NetBSD pre-10, Solaris) may have a non-standard const parameter with iconv. Instead of hardcoding the OS check, move this to a feature check performed by autoconf.

autoconf doesn't have a nicer way of checking this (well, except for AM_ICONV, which is part of gettext and we're presumably not using it), so we have to repeat the function signature and check for it with a mismatched signature.

Tested on NetBSD/amd64 9.4 (for const) and macOS/arm64 14 (for non-const).

[NattyNarwhal]

I added @devnexen and @petk for review since this basically obsoletes GH-16619.

[NattyNarwhal]

I suspect the Windows failure is because AC_DEFINE in the Windows build system will expand an empty string into a "" even if quote is false:

        if (quote && typeof(value) == "string") {
                value = '"' + value.replace(new RegExp('(["\\\\])', "g"), '\\$1') + '"';
        } else if (typeof(value) != "undefined" && value.length == 0) {
                value = '""';
        }

[petk]

I should check this compilation test in more details but something like this can be done here:

• instead of defining a configuration preprocessor macro, a simple compile definition can be added to CFLAGS only in Autotools.

index 4d0dc36489..4aab9945ca 100644
--- a/ext/iconv/config.m4
+++ b/ext/iconv/config.m4
@@ -83,6 +83,18 @@ int main(void) {
   AS_VAR_IF([php_cv_iconv_errno], [yes],,
     [AC_MSG_FAILURE([The iconv check failed, 'errno' is missing.])])
 
+  dnl Some systems (older NetBSD pre-10, Solaris) may have a non-standard iconv
+  dnl signagure with const parameter. libiconv tends to match the system iconv
+  dnl too.
+  AC_CACHE_CHECK([if iconv has non-standard input parameter],
+    [php_cv_iconv_const],
+    [AC_COMPILE_IFELSE([AC_LANG_PROGRAM([
+      #include <iconv.h>
+      size_t iconv(iconv_t cd, const char **src, size_t *srcleft, char **dst, size_t *dstleft);])],
+    [php_cv_iconv_const=yes],
+    [php_cv_iconv_const=no])])
+  AS_VAR_IF([php_cv_iconv_const], [yes], [ICONV_CFLAGS="-DICONV_CONST=const"])
+
   AC_CACHE_CHECK([if iconv supports //IGNORE], [php_cv_iconv_ignore],
     [AC_RUN_IFELSE([AC_LANG_SOURCE([
 #include <iconv.h>
@@ -120,7 +132,7 @@ int main(void) {
   PHP_NEW_EXTENSION([iconv],
     [iconv.c],
     [$ext_shared],,
-    [-DZEND_ENABLE_STATIC_TSRMLS_CACHE=1])
+    [-DZEND_ENABLE_STATIC_TSRMLS_CACHE=1 $ICONV_CFLAGS])
   PHP_SUBST([ICONV_SHARED_LIBADD])
   PHP_INSTALL_HEADERS([ext/iconv], [php_iconv.h])
 fi
diff --git a/ext/iconv/iconv.c b/ext/iconv/iconv.c
index 4241b7c288..e1cb5ef3f5 100644
--- a/ext/iconv/iconv.c
+++ b/ext/iconv/iconv.c
@@ -43,12 +43,8 @@
 #undef iconv
 #endif
 
-#if defined(__NetBSD__)
-// unfortunately, netbsd has still the old non posix conformant signature
-// libiconv tends to match the eventual system's iconv too.
-#define ICONV_CONST const
-#else
-#define ICONV_CONST
+#ifndef ICONV_CONST
+# define ICONV_CONST
 #endif
 
 #include "zend_smart_str.h"

Yes, that GNU lib's iconv check looks like some all-in-one solution with all edge cases considered but it would be also probably a bit over-engineered to add it into PHP: https://git.savannah.gnu.org/cgit/gnulib.git/tree/m4/iconv.m4