Autotools: Fix iconv shared build with external library (#15686)

When building iconv as shared and with external library (for example, libiconv):

    ./configure --with-iconv=shared,/path/to/libiconv

the iconv couldn't be found due to a linker error.

Autoconf places LDFLAGS before the conftest.c file in the test compile
command and LIBS after it. GCC also requires this:

    gcc -L... conftest.c -liconv

Similar issue discovered at
https://github.com/remicollet/php-xpass/pull/1

NEWS

@@ -10,6 +10,8 @@ PHP                                                                        NEWS
   . Fixed bug GH-15330 (Do not scan generator frames more than once). (Arnaud)
   . Fixed bug GH-15644 (Asymmetric visibility doesn't work with hooks). (ilutov)
   . Implemented lazy objects RFC. (Arnaud)
+  . Fixed bug GH-15686 (Building shared iconv with external iconv library).
+    (Peter Kokot, zeriyoshi)
 
 - DOM:
   . Fixed bug GH-13988 (Storing DOMElement consume 4 times more memory in

ext/iconv/config.m4

@@ -8,9 +8,9 @@ if test "$PHP_ICONV" != "no"; then
   PHP_SETUP_ICONV([ICONV_SHARED_LIBADD],,
     [AC_MSG_FAILURE([The iconv not found.])])
 
-  save_LDFLAGS=$LDFLAGS
+  save_LIBS=$LIBS
   save_CFLAGS=$CFLAGS
-  LDFLAGS="$ICONV_SHARED_LIBADD $LDFLAGS"
+  LIBS="$LIBS $ICONV_SHARED_LIBADD"
   CFLAGS="$INCLUDES $CFLAGS"
 
   AC_CACHE_CHECK([for iconv implementation], [php_cv_iconv_implementation], [
@@ -111,7 +111,7 @@ int main(void) {
     [AC_DEFINE([ICONV_BROKEN_IGNORE], [1],
       [Define to 1 if iconv has broken IGNORE.])])
 
-  LDFLAGS=$save_LDFLAGS
+  LIBS=$save_LIBS
   CFLAGS=$save_CFLAGS
 
   AC_DEFINE([HAVE_ICONV], [1],