configure.ac: make msan & ubsan flags additive

[qlyoung]

These were setting = SAN_FLAGS, we want to add to the variable. Note
MSAN can't be used with any other sanitizer (except UBSan) but the
compiler will complain about that if you use it wrong so we don't need
to enforce it here.

Signed-off-by: Quentin Young qlyoung@cumulusnetworks.com

[LabN-CI]

💚 Basic BGPD CI results: SUCCESS, 0 tests failed

Results table

_	_
Result	SUCCESS git merge/5618 982187a
Date	01/03/2020
Start	16:39:41
Finish	17:05:39
Run-Time	25:58
Total	1815
Pass	1815
Fail	0
Valgrind-Errors	0
Valgrind-Loss	0
Details	vncregress-2020-01-03-16:39:41.txt
Log	autoscript-2020-01-03-16:40:38.log.bz2
Memory	421 416 360

For details, please contact louberger

[NetDEF-CI]

Continuous Integration Result: SUCCESSFUL

Congratulations, this patch passed basic tests

Tested-by: NetDEF / OpenSourceRouting.org CI System

CI System Testrun URL: https://ci1.netdef.org/browse/FRR-FRRPULLREQ-10279/

This is a comment from an automated CI system.
For questions and feedback in regards to this CI system, please feel free to email
Martin Winter - mwinter (at) opensourcerouting.org.

Warnings Generated during build:

Debian 10 amd64 build: Successful with additional warnings

Debian Package lintian failed for Debian 10 amd64 build:
(see full package build log at https://ci1.netdef.org/browse/FRR-FRRPULLREQ-10279/artifact/DEB10BUILD/ErrorLog/log_lintian.txt)

W: frr source: pkg-js-tools-test-is-missing
W: frr source: pkg-js-tools-test-is-missing
W: frr-doc: changelog-file-missing-explicit-entry 6.0-2 -> 7.3-dev-20200103-05-g982187afb-0 (missing) -> 7.3-dev-20200103-05-g982187afb-0~deb10u1
W: frr: changelog-file-missing-explicit-entry 6.0-2 -> 7.3-dev-20200103-05-g982187afb-0 (missing) -> 7.3-dev-20200103-05-g982187afb-0~deb10u1
W: frr: spelling-error-in-readme-debian explecitly explicitly
W: frr-rpki-rtrlib: changelog-file-missing-explicit-entry 6.0-2 -> 7.3-dev-20200103-05-g982187afb-0 (missing) -> 7.3-dev-20200103-05-g982187afb-0~deb10u1
W: frr-pythontools: changelog-file-missing-explicit-entry 6.0-2 -> 7.3-dev-20200103-05-g982187afb-0 (missing) -> 7.3-dev-20200103-05-g982187afb-0~deb10u1
W: frr-snmp: changelog-file-missing-explicit-entry 6.0-2 -> 7.3-dev-20200103-05-g982187afb-0 (missing) -> 7.3-dev-20200103-05-g982187afb-0~deb10u1