CI: travis-ci: use proper linking flags for SLZ build

previously SSL_INC and SSL_INC were set for all builds, and SLZ lib
was linked because of those flags. After we switched SLZ build to
stock openssl lib, SSL_INC, SSL_LIB are not set anymore.

Good time to set SLZ_INC, SLZ_LIB for such builds
diff --git a/.travis.yml b/.travis.yml
index 2d2a027..ca6073e 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -110,6 +110,8 @@
     before_script:
       - git clone https://github.com/wtarreau/libslz
       - cd libslz && make && make PREFIX=${HOME}/opt install && cd ..
+      - export SLZ_INC=${HOME}/opt/include SLZ_LIB=${HOME}/opt/lib
+      - export ADDLIB="-Wl,-rpath,$SLZ_LIB"
     name: openssl-1.1.1 | slz | pcre2
   allow_failures:
   - os: linux
@@ -125,12 +127,13 @@
     if [ ! -z ${OPENSSL_VERSION+x} ] || [ ! -z ${LIBRESSL_VERSION+x} ] || [ ! -z ${BORINGSSL+x} ]; then
        travis_wait bash -c 'scripts/build-ssl.sh >build-ssl.log 2>&1' || (cat build-ssl.log && exit 1)
        export SSL_LIB=${HOME}/opt/lib SSL_INC=${HOME}/opt/include
+       export ADDLIB="-Wl,-rpath,$SSL_LIB"
     fi
 
 script:
   - if [ "${CC%-*}"  = "clang" ]; then export FLAGS="$FLAGS USE_OBSOLETE_LINKER=1" DEBUG_CFLAGS="-g -fsanitize=address" LDFLAGS="-fsanitize=address"; fi
   - make -C contrib/wurfl
-  - make -j3 CC=$CC CPU_CFLAGS.generic="-O1" V=1 ERR=1 TARGET=$TARGET $FLAGS DEBUG_CFLAGS="$DEBUG_CFLAGS" LDFLAGS="$LDFLAGS" $([ ! -z ${SSL_LIB+x} ] && echo ADDLIB="-Wl,-rpath,$SSL_LIB") 51DEGREES_SRC="$FIFTYONEDEGREES_SRC" EXTRA_OBJS="$EXTRA_OBJS" $DEBUG_OPTIONS
+  - make -j3 CC=$CC CPU_CFLAGS.generic="-O1" V=1 ERR=1 TARGET=$TARGET $FLAGS DEBUG_CFLAGS="$DEBUG_CFLAGS" LDFLAGS="$LDFLAGS" ADDLIB="$ADDLIB" 51DEGREES_SRC="$FIFTYONEDEGREES_SRC" EXTRA_OBJS="$EXTRA_OBJS" $DEBUG_OPTIONS
   - ./haproxy -vv
   - if [ "${TRAVIS_OS_NAME}" = "linux" ]; then ldd haproxy; fi
   - if [ "${TRAVIS_OS_NAME}" = "osx" ]; then otool -L haproxy; fi