configure: add support for libdir option

This series add support for the libdir parameter in iproute2 configure script. The idea is to make use of the fact that packaging systems may assume that 'configure' comes from autotools allowing a syntax similar to the autotools one, and using it to tell iproute2 where the distro expects to find its lib files.

Patches 1-2 fix a parsing issue on current configure options, that may trigger an endless loop when no value is provided with some options;

Patch 3 fixes a parsing issue bailing out when more than one value is provided for a single option;

Patch 4 simplifies options parsing, moving semantic checks out of the while loop processing options;

Patch 5 introduces support for the --opt=value style on current options, for uniformity;

Patch 6 adds the --prefix option, that may be used by some packaging systems when calling the configure script;

Patch 7 finally adds the --libdir option, and also drops the static LIBDIR var from the Makefile.

Changelog:
----------
v4 -> v5
- bail out when multiple values are provided with a single option
- simplify option parsing and reduce code duplication, as suggested by Phil Sutter
- remove a nasty eval on libdir option processing

v3 -> v4
- fix parsing issue on '--include_dir' and '--libbpf_dir'
- split '--opt value' and '--opt=value' use cases, avoid code duplication moving semantic checks on value to dedicated functions

v2 -> v3
- fix parsing error on prefix and libdir options.

v1 -> v2
- consolidate '--opt value' and '--opt=value' use cases, as suggested by David Ahern.
- added patch 2 to manage the --prefix option, used by the Debian packaging system, as reported by Luca Boccassi, and use it when setting lib directory.

Andrea Claudi (7):
configure: fix parsing issue on include_dir option
configure: fix parsing issue on libbpf_dir option
configure: fix parsing issue with more than one value per option
configure: simplify options parsing
configure: support --param=value style
configure: add the --prefix option
configure: add the --libdir option

 Makefile  |  7 ++---
 configure | 78 +++++++++++++++++++++++++++++++++++++++++--------------
 2 files changed, 63 insertions(+), 22 deletions(-)

2.31.1