configure uses incorrect link order when testing libpython

References:

https://stackoverflow.com/a/49868387
https://sourceware.org/bugzilla/show_bug.cgi?id=11420

Configure uses "gcc -o conftest -g ... conftest.c -ldl -lncurses -lm -ldl
... -lpthread ... -lpython2.7" when deciding whether give libpython is
usable.

That of course is the wrong link order, and only works for shared libraries
(mostly by accident), and only on some systems.

gdb/ChangeLog:

	PR gdb/11420
	* configure.ac: Prepend libpython.
	* python/python-config.py: Likewise.
	* configure: Regenerate.

gdb/python/python-config.py

@@ -58,12 +58,11 @@ for opt in opt_flags:
         print (to_unix_path(' '.join(flags)))
 
     elif opt in ('--libs', '--ldflags'):
-        libs = []
+        libs = ['-lpython' + pyver + abiflags]
         if getvar('LIBS') is not None:
             libs.extend(getvar('LIBS').split())
         if getvar('SYSLIBS') is not None:
             libs.extend(getvar('SYSLIBS').split())
-        libs.append('-lpython'+pyver + abiflags)
         # add the prefix/lib/pythonX.Y/config dir, but only if there is no
         # shared library in prefix/lib/.
         if opt == '--ldflags':