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/Makefile.in ltrace-0.4/Makefile.in

 --- ltrace-0.4.orig/Makefile.in	2008-10-26 15:34:21.000000000 +0100

 +++ ltrace-0.4/Makefile.in	2008-10-26 15:37:40.000000000 +0100

 @@ -5,6 +5,7 @@

  OS		:= @HOST_OS@

  ARCH		:= $(shell echo "@HOST_ARCH@" |sed -e s/i.86/i386/      \

  						   -e s/sun4u/sparc64/  \

 +						   -e s/alpha.*/alpha/  \

  						   -e s/arm.*/arm/      \

  						   -e s/sa110/arm/      \

  						   -e s/ppc64/ppc/      \