Search gcc search path for dbm first

It's possible to persuade GCC to reveal its search path for header
files, which provides a better default for systems compiling with gcc.
It also means that the mingw-w64 libraries are automatically detected.

configure

@@ -50,7 +50,7 @@ dbm_include="not found"
 dbm_link="not found"
 dbm_defines=""
 
-for dir in /usr/include /usr/include/db1 /usr/include/gdbm /usr/local/include; do
+for dir in `$CC -xc -E -v /dev/null 2>&1 | sed -e '1,/#include <...>/d' -e '/End of search list./,$d'` /usr/include /usr/include/db1 /usr/include/gdbm /usr/local/include ; do
   if test -f $dir/ndbm.h; then
     dbm_include=$dir
     dbm_defines="-DDBM_COMPAT"