configure: non pkg-config fallback ignores custom CFLAGS and LIBS

**Bug report**

In ``configure.ac`` we have several fallbacks when pkg-config is missing or a package does not provide a ``.pc`` file. The fallbacks do not take custom cflags into account. For example the block for ``zlib`` does ``AC_CHECK_HEADERS([zlib.h])`` and ``AC_CHECK_LIB([z])`` without setting ``CFLAGS="$CFLAGS $ZLIB_CFLAGS"`` first.