Autotools: Fix iconv shared build with external library

When building:

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

the iconv couldn't be found due to linker error

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

Similar issue discovered at
remicollet/php-xpass#1

Author: petk

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],