Fix Makefile for BSD-make

The usual GNU-extension stuff: `$(shell ..)` is an extension that is not
supported by BSD-make
Simply changed it into an inline-evaluation of pkg-config within each
make target.

Makefile

@@ -1,10 +1,10 @@
 VODUS_PKGS=freetype2 libavcodec libavutil
-VODUS_CXXFLAGS=-Wall -fno-exceptions -std=c++17 -ggdb $(shell pkg-config --cflags $(VODUS_PKGS))
-VODUS_LIBS=$(shell pkg-config --libs $(VODUS_PKGS)) -lgif
+VODUS_CXXFLAGS=-Wall -fno-exceptions -std=c++17 -ggdb `pkg-config --cflags $(VODUS_PKGS)`
+VODUS_LIBS=`pkg-config --libs $(VODUS_PKGS)` -lgif
 
 EMOTE_DOWNLOADER_PKGS=libcurl
-EMOTE_DOWNLOADER_CXXFLAGS=-Wall -fno-exceptions -std=c++17 -ggdb $(shell pkg-config --cflags $(EMOTE_DOWNLOADER_PKGS))
-EMOTE_DOWNLOADER_LIBS=$(shell pkg-config --libs $(EMOTE_DOWNLOADER_PKGS))
+EMOTE_DOWNLOADER_CXXFLAGS=-Wall -fno-exceptions -std=c++17 -ggdb `pkg-config --cflags $(EMOTE_DOWNLOADER_PKGS)`
+EMOTE_DOWNLOADER_LIBS=`pkg-config --libs $(EMOTE_DOWNLOADER_PKGS)`
 
 RENDER_ARGS=--font assets/ComicNeue_Bold.otf --output output.mpeg --font-size 46 --fps 30 --width 600 --height 1080 --limit 20 sample.txt --bitrate 6000000