Slackbuild:How to (elegantly) add a path for linking

I'm trying to install the last installment of Flightgear, and get this error on linking:

Code:[100%] Linking CXX executable fgfs
/usr/bin/ld: /usr/lib64/gcc/x86_64-slackware-linux/10.3.0/../../../../lib64/libflite.so: undefined reference to `snd_pcm_open'
/usr/bin/ld: /usr/lib64/gcc/x86_64-slackware-linux/10.3.0/../../../../lib64/libflite.so: undefined reference to `snd_pcm_prepare'
/usr/bin/ld: /usr/lib64/gcc/x86_64-slackware-linux/10.3.0/../../../../lib64/libflite.so: undefined reference to `snd_pcm_close'
/usr/bin/ld: /usr/lib64/gcc/x86_64-slackware-linux/10.3.0/../../../../lib64/libflite.so: undefined reference to `snd_pcm_drop'
...
/usr/bin/ld: /usr/lib64/gcc/x86_64-slackware-linux/10.3.0/../../../../lib64/libflite.so: undefined reference to `snd_pcm_writei'
/usr/bin/ld: /usr/lib64/gcc/x86_64-slackware-linux/10.3.0/../../../../lib64/libflite.so: undefined reference to `snd_pcm_resume'
collect2: error: ld returned 1 exit statusAccording to comments, a solution would be to add -lasound to LIBS.

What would be the 'slackbuilds' way to do this? Create a patch?