glibc: properly handle internal addons

Some addons are bundled with glibc/eglibc, so we should not try to
download and extract them.

This is done as thus:
- at download time:
- if the add-on download fails, keep going;
- at extract time:
- if the addon is present in the source tree, ignore it;
- if the addon is missing in the source tree:
- if the archive is present, extract it;
- if the archive is missing, bail out.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 # dmalloc debug facility

 ## depends on ! BACKEND

 ##

 ## help dmalloc helps tracking memory allocation mis-usage

 ## help such as double-free, use-after-free...

 choice

     bool

     prompt "dmalloc version"

 # Don't remove next line

 # CT_INSERT_VERSION_BELOW

 config DMALLOC_V_5_5_2

     bool

     prompt "5.5.2"

 endchoice

 config DMALLOC_VERSION

     string

 # Don't remove next line

 # CT_INSERT_VERSION_STRING_BELOW

     default "5.5.2" if DMALLOC_V_5_5_2