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)

--- gcc-4.1.0/libstdc++-v3/include/ext/rope.mps	2006-03-24 01:49:51 +0100

+++ gcc-4.1.0/libstdc++-v3/include/ext/rope	2006-03-24 01:49:37 +0100

@@ -59,6 +59,9 @@

 #include <bits/allocator.h>

 #include <ext/hash_fun.h>

+/* cope w/ index defined as macro, SuSv3 proposal */

+#undef index

+

 # ifdef __GC

 #   define __GC_CONST const

 # else

--- gcc-4.1.0/libstdc++-v3/include/ext/ropeimpl.h.mps	2006-03-24 01:50:04 +0100

+++ gcc-4.1.0/libstdc++-v3/include/ext/ropeimpl.h	2006-03-24 01:50:28 +0100

@@ -53,6 +53,9 @@

 #include <ext/memory> // For uninitialized_copy_n

 #include <ext/numeric> // For power

+/* cope w/ index defined as macro, SuSv3 proposal */

+#undef index

+

 namespace __gnu_cxx

 {

   using std::size_t;