scripts: remove . from $PATH

Add CT_SanitizePath function which removes entries referring to ., /tmp
and non-existing directories from $PATH, and call it early in the
build script.

If . is in PATH, gcc-4.4.4 build breaks:

[ALL ] checking what assembler to use...
/tmp/build/targets/arm-unknown-linux-uclibcgnueabi/build/gcc-core-static/arm-unknown-linux-uclibcgnueabi/bin/as
...
[ALL ] config.status: creating as

i.e. "as" is supposed to be the arm-unknown-linux-uclibcgnueabi cross assembler,
but config.status creates a local "as" script which is calling the
host assembler.

Signed-off-by: Johannes Stezenbach <js@sig21.net>
[Yann E. MORIN: style fixes + explanations]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 20dd8cef1c8adff0aa3e78ae6d7acfbc45ed5a83)

diff -durN gcc-4.4.0.orig/include/libiberty.h gcc-4.4.0/include/libiberty.h

--- gcc-4.4.0.orig/include/libiberty.h	2008-06-24 03:42:31.000000000 +0200

+++ gcc-4.4.0/include/libiberty.h	2009-05-27 21:38:24.000000000 +0200

@@ -595,8 +595,11 @@

 /* Like sprintf but provides a pointer to malloc'd storage, which must

    be freed by the caller.  */

+/* asprintf may be declared as a macro by glibc with __USE_FORTIFY_LEVEL.  */

+#ifndef asprintf

 extern int asprintf (char **, const char *, ...) ATTRIBUTE_PRINTF_2;

 #endif

+#endif

 #if !HAVE_DECL_VASPRINTF

 /* Like vsprintf but provides a pointer to malloc'd storage, which