graphics/sdl_ttf: unbreak --no-allow-shlib-undefined with Gold or LLD 9

Clang 8 on x86 with -O1 converts ceil(float) to ceilf() but wants
-msse4.1 to use builtin. GCC 8 on x86 and aarch64 with -O1 and Clang 8
on aarch64 always use builtin ceil(). To avoid guessing when library
call will be made just append -lm explicitly.

ld: error: ./.libs/libSDL_ttf.so: undefined reference to ceilf

PR: 236141
Reported by: pkg-fallout (via antoine)
Obtained from: upstream
Approved by: portmgr blanket (blocks 144 consumers)