scripts, cc/gcc: do not fail on existing symlinks or build.log

If the user builds a toolchain over an existing one, so, without removing
CT_PREFIX_DIR, the build fails as the symlinks already exist, as does the
build.log.

This can also happen (for build.log) if the user first ran in download-
or extract-only.

Patch (with no SoB) originally from:
Phil Wilshire <phil.wilshire@overturenetworks.com>

Modified by me as it did not apply cleanly.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from 1ebc2248cc60230cd53ff94ae8f8f1e3261461a3)

 http://sourceware.org/ml/binutils/2007-07/msg00401.html

 http://sourceware.org/bugzilla/show_bug.cgi?id=4970

 --- binutils/configure

 +++ binutils/configure

 @@ -5601,6 +5601,7 @@ case "${host}" in

    *-*-mingw* | *-*-cygwin ) RPATH_ENVVAR=PATH ;;

    *) RPATH_ENVVAR=LD_LIBRARY_PATH ;;

  esac

 +RPATH_ENVVAR="cant_touch_this_nah_nah_nah"

  # On systems where the dynamic library environment variable is PATH,

  if test "$RPATH_ENVVAR" = PATH; then