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)

 do not bother running ldconfig on DESTDIR.  it wants to write the temp cache

 file outside of the chroot.  doesnt matter anyways as we wont use the cache

 results (portage will rebuild cache), so running ldconfig is simply a waste

 of time.

 diff -durN glibc-2.10.1.orig/Makefile glibc-2.10.1/Makefile

 --- glibc-2.10.1.orig/Makefile	2009-04-23 20:23:03.000000000 +0200

 +++ glibc-2.10.1/Makefile	2009-11-13 00:49:49.000000000 +0100

 @@ -116,6 +116,7 @@

  	rm -f $(symbolic-link-list)

  install:

 +dont-bother-with-destdir:

  	-test ! -x $(common-objpfx)elf/ldconfig || LC_ALL=C LANGUAGE=C \

  	  $(common-objpfx)elf/ldconfig $(addprefix -r ,$(install_root)) \

  				       $(slibdir) $(libdir)