Ignore pkg-config error messages

[yamt]

To avoid errors like the following:

cc -Ilib -O3 -Wall -fPIC Package sdl was not found in the pkg-config search path. Perhaps you should add the directory containing `sdl.pc' to the PKG_CONFIG_PATH environment variable Package 'sdl' not found -o lib/decode.o -c lib/decode.c
/bin/sh: -c: line 0: unexpected EOF while looking for matching ``'
/bin/sh: -c: line 1: syntax error: unexpected end of file
gmake: *** [Makefile:90: lib/decode.o] Error 2

This (at least the SDL part) was neccessary on my environment:

• macOS 15.2
• Xcode 16.2
• pkg-config 2.3.0 (homebrew)
• GNU make 4.4.1 (homebrew)
• opencv 4.11.0 (homebrew)
• no SDL installed