build.sh: on macOS, help compiling tools with correct path configuration

When installing libpng (used by mksprite) via homebrew, we need to
export the correct paths in CFLAGS/LDFLAGS

build.sh

@@ -4,6 +4,20 @@
 set -euo pipefail
 IFS=$'\n\t'
 
+if [[ -z ${N64_INST-} ]]; then
+  echo N64_INST environment variable is not defined
+  echo Please set N64_INST to point to your libdragon toolchain directory
+  exit 1
+fi
+
+if [[ $OSTYPE == 'darwin'* ]]; then
+  if command -v brew >/dev/null; then
+    brew install libpng
+    CFLAGS="-I$(brew --prefix)/include"; export CFLAGS
+    LDFLAGS="-L$(brew --prefix)/lib"; export LDFLAGS
+  fi
+fi
+
 makeWithParams(){
   make -j"${JOBS}" "$@" || \
     sudo env N64_INST="$N64_INST" CFLAGS="$CFLAGS" LDFLAGS="$LDFLAGS" \

tools/mksprite/Makefile

@@ -1,6 +1,6 @@
 INSTALLDIR = $(N64_INST)
-CFLAGS = -std=gnu99 -O2 -Wall -Werror -Wno-unused-result -I../../include
-LDFLAGS = -lpng
+CFLAGS += -std=gnu99 -O2 -Wall -Werror -Wno-unused-result -I../../include
+LDFLAGS += -lpng
 all: mksprite convtool
 
 mksprite: