Inject SDL2 specific flags into variables via pkgconf when available

SDL2 is only available at a non-default path on some systems. pkgconf, when installed, may help in those cases

build.sh

@@ -1,14 +1,14 @@
 #!/usr/bin/env bash
 
 cflags="-Wall -O3 -g -std=gnu11 -Isrc"
-lflags="-lSDL2 -lm"
+lflags="-lm"
 
 if [[ $* == *windows* ]]; then
   platform="windows"
   outfile="lite.exe"
   compiler="x86_64-w64-mingw32-gcc"
   cflags="$cflags -DLUA_USE_POPEN -Iwinlib/SDL2-2.0.10/x86_64-w64-mingw32/include"
-  lflags="$lflags -Lwinlib/SDL2-2.0.10/x86_64-w64-mingw32/lib"
+  lflags="$lflags -lSDL2 -Lwinlib/SDL2-2.0.10/x86_64-w64-mingw32/lib"
   lflags="-lmingw32 -lSDL2main $lflags -mwindows -o $outfile res.res"
   x86_64-w64-mingw32-windres res.rc -O coff -o res.res
 else
@@ -17,6 +17,12 @@ else
   compiler="cc"
   cflags="$cflags -DLUA_USE_POSIX"
   lflags="$lflags -o $outfile"
+  if command -v pkgconf >/dev/null; then
+    cflags="$cflags $(pkgconf --cflags --silence-errors sdl2)"
+    lflags="$lflags $(pkgconf --libs --silence-errors sdl2)"
+  else
+    lflags="$lflags -lSDL2"
+  fi
 fi
 
 if command -v ccache >/dev/null; then