[BUILD] Fixed Makefile for linking pcre

[BUILD] Fixed Makefile for linking pcre

If both make parameters USE_PCRE and USE_STATIC_PCRE are set to 1
while building haproxy, pcre gets linked in dynamically.

Therefore we check if USE_STATIC_PCRE was explicitely enabled to
ommit the CFLAGS and LDFLAGS normally set if USE_PCRE is enabled.
(cherry picked from commit c820300adf36700cbbff15249a8629baa63c5f5a)