Rename contrib/pngminim/*/makefile.std to makefile

since there's only one makefile, so users can just type "make"
instead of "make -f makefile.std".

contrib/pngminim/encoder/README

@@ -1,6 +1,7 @@
 This demonstrates the use of PNG_USER_CONFIG and pngusr.h
 
 To build a minimal write-only encoder, run
+
     gather.sh  # to collect needed files from pngminus, libpng, and zlib
-    make -f makefile.std
+    make
  

contrib/pngminim/encoder/makefile.std

@@ -1,43 +0,0 @@
-# Makefile for PngMinus (pnm2pngm)
-# Linux / Unix
-
-#CC=cc
-CC=gcc
-LD=$(CC)
-
-RM=rm -f
-
-CFLAGS=-DPNG_USER_CONFIG -DNO_GZIP -I. -O1
-
-C=.c
-O=.o
-L=.a
-E=
-
-ZOBJS  = adler32$(O) compress$(O) crc32$(O) deflate$(O) gzio$(O) \
-	 dummy_inflate$(O) \
-	 trees$(O) uncompr$(O) zutil$(O)
-
-OBJS  = pnm2pngm$(O) png$(O) pngerror$(O) pngget$(O) pngmem$(O) \
-	pngset$(O) pngtrans$(O) pngwio$(O) pngwrite$(O) \
-	pngwtran$(O) pngwutil$(O) $(ZOBJS)
-
-# implicit make rules -------------------------------------------------------
-
-.c$(O): png.h pngconf.h pngusr.h zlib.h
-	$(CC) -c $(CFLAGS) $<
-
-# dependencies
-
-all: pnm2pngm$(E)
-
-pnm2pngm$(E): $(OBJS)
-	$(LD) -o pnm2pngm$(E) $(OBJS)
-	strip pnm2pngm$(E)
-
-clean:
-	$(RM) pnm2pngm$(O)
-	$(RM) pnm2pngm$(E)
-	$(RM) $(OBJS)
-
-# End of makefile for pnm2pngm