kernel/linux: fix using custom location

Currently, extract and patch are skipped as thus:
- using a custom directory of pre-installed headers
- a correctly named directory already exists

Otherwise, extract and patch are done.

The current second condition is wrong, because it allows the following
sequence to happen:
- a non-custom kernel is used
- a previous build only partially extracted the non-custom sources
- that p[revious build broke during extraction (eg. incomplete tarball...)
- a subsequent build will find a properly named directory, and will
thus skip extract and patch, which is wrong

Fix that by following the conditions in this table:

Type | Extract | Patch
----------------------+---------+-------
Pre-installed headers | N | N
custom directory | N | N
custom tarball | Y | N
mainstream tarball | Y | Y

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: David Holsgrove <david.holsgrove@xilinx.com>

 diff -Naurd cloog-ppl-0.15.10-a/configure cloog-ppl-0.15.10-b/configure

 --- cloog-ppl-0.15.10-a/configure	2010-08-27 21:14:33.000000000 +0200

 +++ cloog-ppl-0.15.10-b/configure	2012-01-27 13:28:49.262987773 +0100

 @@ -11168,7 +11168,7 @@

  fi

  rm -f core conftest.err conftest.$ac_objext conftest.$ac_ext

 -	LIBS="$LIBS -lppl_c -lppl -lgmpxx"

 +	LIBS="-lppl_c -lppl -lgmpxx $LIBS"

  $as_echo "#define CLOOG_PPL_BACKEND 1" >>confdefs.h

 diff -Naurd cloog-ppl-0.15.10-a/configure.in cloog-ppl-0.15.10-b/configure.in

 --- cloog-ppl-0.15.10-a/configure.in	2010-08-27 21:13:48.000000000 +0200

 +++ cloog-ppl-0.15.10-b/configure.in	2012-01-27 13:28:49.262987773 +0100

 @@ -343,7 +343,7 @@

  	#endif

  	], [AC_MSG_RESULT([yes])], [AC_MSG_ERROR(Can't find correct version of PPL.) ])

 -	LIBS="$LIBS -lppl_c -lppl -lgmpxx"

 +	LIBS="-lppl_c -lppl -lgmpxx $LIBS"

  	AC_DEFINE([CLOOG_PPL_BACKEND], 1, [Use the PPL backend])

  else