First part has been submitted upstream:
https://github.com/kornelski/pngquant/pull/303

diff -u -r ../pngquant-2.12.1/configure ./configure
--- ../pngquant-2.12.1/configure	2018-06-04 11:30:47.000000000 +0000
+++ ./configure	2018-07-02 11:53:11.547417517 +0000
@@ -159,10 +159,11 @@
 
 find_pkgconfig() {
     local LIBNAME=$1
-    if pkg-config --exists "$LIBNAME" &> /dev/null; then
-        cflags "$(pkg-config --cflags "$LIBNAME")"
-        lflags "$(pkg-config --libs "$LIBNAME")"
-        LIBRARY_FOUND_VERSION=$(pkg-config --modversion "$LIBNAME")
+    PKG_CONFIG=${PKG_CONFIG:-pkg-config}
+    if $PKG_CONFIG --exists "$LIBNAME" &> /dev/null; then
+        cflags "$($PKG_CONFIG --cflags "$LIBNAME")"
+        lflags "$($PKG_CONFIG --libs "$LIBNAME")"
+        LIBRARY_FOUND_VERSION=$($PKG_CONFIG --modversion "$LIBNAME")
         status "$LIBNAME" "shared ($LIBRARY_FOUND_VERSION)"
         return 0
     fi
@@ -360,12 +361,6 @@
     DIRS+=("$LIQSRCDIR" "$LIQSRCDIR") # local libimagequant
 fi
 
-DIRS+=(
-      "/usr/local/include /usr/local/lib"
-      "/usr/include /usr/lib64"
-      "/usr/include /usr/lib"
-      "/opt/local/include /opt/local/lib" # macports
-      )
 
 if [[ "$OSTYPE" =~ "darwin" ]]; then
     SOLIBSUFFIX=dylib