Makefile: Fix library link order

When compiling with the internal lexbor, the linker is invoked as
cc [...] -lm lexbor_static.a which, at least on i386 glibc, fails with:

/usr/bin/ld: c_src/lexbor/liblexbor_static.a(diyfp.c.o): in function `lexbor_cached_power_bin':
./deps/fast_html/c_src/lexbor/./deps/fast_html/c_src/lexbor/source/lexbor/core/diyfp.c:140: undefined reference to `ceil'
collect2: error: ld returned 1 exit status
make[2]: *** [Makefile:45: priv/fasthtml_worker] Error 1

Fix it by ordering -lm after lexbor_static.a.