debug/gdb: properly link with expat

This patch fixes libexpat detection for gdb-native and
gdb-cross static builds.

For gdb-native build configure should not touch system
/usr/{lib,include} directories while looking for libexpat.
To fix this we pass --without-libexpat-prefix flag
to configure script.

For gdb-cross build configure is allowed to investigate
system /usr/{lib,include} directories, but it does not
hurt to disable this behavior. In this case configure
falls back to -lexpat, which works as expected.

For more info:
http://marc.info/?l=gnulib-bug&m=129660262901148&w=2

Signed-off-by: Kirill Smirnov <kirill.k.smirnov@gmail.com>

scripts/build/debug/300-gdb.sh

@@ -69,11 +69,8 @@ do_debug_gdb_build() {
 
         cross_extra_config=("${extra_config[@]}")
         cross_extra_config+=("--with-expat")
-        # NOTE: DO NOT USE --with-libexpat-prefix (until GDB configure is smarter)!!!
-        # It conflicts with a static build: GDB's configure script will find the shared
-        # version of expat and will attempt to link that, despite the -static flag.
-        # The link will fail, and configure will abort with "expat missing or unusable"
-        # message.
+        cross_extra_config+=("--without-libexpat-prefix")
+
         case "${CT_THREADS}" in
             none)   cross_extra_config+=("--disable-threads");;
             *)      cross_extra_config+=("--enable-threads");;
@@ -173,11 +170,7 @@ do_debug_gdb_build() {
         fi
 
         native_extra_config+=("--with-expat")
-        # NOTE: DO NOT USE --with-libexpat-prefix (until GDB configure is smarter)!!!
-        # It conflicts with a static build: GDB's configure script will find the shared
-        # version of expat and will attempt to link that, despite the -static flag.
-        # The link will fail, and configure will abort with "expat missing or unusable"
-        # message.
+        native_extra_config+=("--without-libexpat-prefix")
 
         CT_DoLog EXTRA "Configuring native gdb"