It looks like moving the patch after .include <bsd.port.pre.mk> made it not work in a way that I didn't notice yesterday.
This was the error message, maybe due to BUILD_DEPENDS= replacing that from USES=gmake:
**************************************************** Configuration done, ready to 'make install' (see README and INSTALL files for more info.) **************************************************** cd /wrkdirs/usr/ports/lang/ghc/work/ghc-8.6.5-boot && PACKAGES='' gmake install /bin/sh: gmake: not found *** Error code 127 Stop. make: stopped in /usr/ports/lang/ghc =>> Cleaning up wrkdir ===> Cleaning for ghc-8.6.5_5 build of lang/ghc | ghc-8.6.5_5 ended at Wed Mar 11 10:55:58 CET 2020 build time: 00:00:22 !!! build failure encountered !!