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-0.5.2.orig/configure ltrace-0.5.2/configure

 --- ltrace-0.5.2.orig/configure	2009-09-14 18:47:45.000000000 +0200

 +++ ltrace-0.5.2/configure	2009-09-14 18:49:18.000000000 +0200

 @@ -1829,6 +1829,9 @@

    linux-gnu*) host_os=linux-gnu

  esac

  HOST_OS="$host_os"

 +case "${HOST_OS}" in

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

 +esac

  ac_ext=c

 diff -durN ltrace-0.5.2.orig/configure.ac ltrace-0.5.2/configure.ac

 --- ltrace-0.5.2.orig/configure.ac	2009-09-14 18:47:45.000000000 +0200

 +++ ltrace-0.5.2/configure.ac	2009-09-14 18:49:18.000000000 +0200

 @@ -10,6 +10,9 @@

    linux-gnu*) host_os=linux-gnu

  esac

  HOST_OS="$host_os"

 +case "${HOST_OS}" in

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

 +esac

  AC_SUBST(HOST_OS)

  HOST_ARCH="$host_cpu"

  AC_SUBST(HOST_ARCH)