also add include parameter for checking iconv library, or libc and gnulibiconv might get mixed

configure

@@ -815,6 +815,7 @@ with_ffi_include
 with_ffi_library
 enable_iconv
 with_libiconv_library
+with_libiconv_include
 enable_stricticonv
 enable_xml
 with_xml_prefix
@@ -1582,6 +1583,7 @@ Optional Packages:
   --with-ffi-include=PATH       Include path for ffi headers
   --with-ffi-library=PATH       Library path for ffi libs
   --with-libiconv-library=PATH  Library path for libiconv libraries
+  --with-libiconv-include=PATH  Include path for libiconv header
   --with-xml-prefix=PFX    Prefix where libxml is installed (optional)
   --with-tls-prefix=PFX    Prefix where libgnutls is installed (optional)
   --with-zeroconf-api=API  force use of a specific zeroconf API (mdns or avahi)
@@ -11531,6 +11533,19 @@ esac
 
   fi
 
+
+# Check whether --with-libiconv-include was given.
+if test "${with_libiconv_include+set}" = set; then :
+  withval=$with_libiconv_include; libiconv_incdir="$withval"
+else
+  libiconv_incdir="no"
+fi
+
+  if test ${libiconv_incdir} != "no"; then
+      CPPFLAGS="$CPPFLAGS -I${libiconv_incdir}"
+      INCLUDE_FLAGS="$INCLUDE_FLAGS -I${libiconv_incdir}"
+  fi
+
   old_LIBS="$LIBS"
   LIBS="-liconv $LIBS"
   if test "$cross_compiling" = yes; then :

configure.ac

@@ -3169,6 +3169,14 @@ if test $found_iconv = no ; then
     GS_ADD_LIBRARY_PATH([${libiconv_libdir}])
   fi
 
+  AC_ARG_WITH(libiconv-include,
+    [  --with-libiconv-include=PATH  Include path for libiconv header],
+    libiconv_incdir="$withval", libiconv_incdir="no")
+  if test ${libiconv_incdir} != "no"; then
+      CPPFLAGS="$CPPFLAGS -I${libiconv_incdir}"
+      INCLUDE_FLAGS="$INCLUDE_FLAGS -I${libiconv_incdir}"
+  fi
+
   old_LIBS="$LIBS"
   LIBS="-liconv $LIBS"
   AC_RUN_IFELSE([AC_LANG_SOURCE([#include <iconv.h>