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 ltrace-0.4.orig/configure ltrace-0.4/configure

 --- ltrace-0.4.orig/configure	2008-10-23 23:19:38.000000000 +0200

 +++ ltrace-0.4/configure	2008-10-23 23:19:06.000000000 +0200

 @@ -1392,6 +1392,9 @@

  HOST_OS="$host_os"

 +case "${HOST_OS}" in

 +  linux-*) HOST_OS=linux-gnu;;

 +esac

  ac_ext=c

 diff -durN ltrace-0.4.orig/configure.ac ltrace-0.4/configure.ac

 --- ltrace-0.4.orig/configure.ac	2008-10-23 23:19:43.000000000 +0200

 +++ ltrace-0.4/configure.ac	2008-10-23 23:18:44.000000000 +0200

 @@ -7,6 +7,9 @@

  dnl Check host system type

  AC_CANONICAL_HOST

  HOST_OS="$host_os"

 +case "${HOST_OS}" in

 +  linux-*) HOST_OS=linux-gnu;;

 +esac

  AC_SUBST(HOST_OS)

  dnl Checks for programs.