[osx_steps] export cppflags

Prefer multibuild library locations over the defaults.
Sometimes multibuild would use an older version
over it's own.

tests/test_library_builders.sh

@@ -21,7 +21,7 @@ function suppress {
 if [ "$TRAVIS_OS_NAME" == "osx" ]; then brew update; fi
 
 suppress build_openssl
-suppress build_openblas
+suppress build_libpng
 suppress build_libwebp
 suppress build_szip
 suppress build_swig

travis_steps.sh

@@ -10,3 +10,7 @@ if [[ "$TRAVIS_OS_NAME" == "osx" ]]; then
 else
     source $MULTIBUILD_DIR/travis_linux_steps.sh
 fi
+
+# Promote BUILD_PREFIX on search path to find new zlib
+export CPPFLAGS="-L$BUILD_PREFIX/include $CPPFLAGS"
+export LDFLAGS="-L$BUILD_PREFIX/lib $LDFLAGS"