Set the library soname to libpng instead of libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@.

Index: Makefile.in
--- Makefile.in.orig
+++ Makefile.in
@@ -834,7 +834,7 @@ EXTRA_SCRIPTS = libpng-config libpng@PNGLIB_MAJOR@@PNG
 bin_SCRIPTS = @binconfigs@
 
 # rules to build libpng, only build the old library on request
-lib_LTLIBRARIES = libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@.la
+lib_LTLIBRARIES = libpng.la
 # EXTRA_LTLIBRARIES= libpng.la
 libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@_la_SOURCES = png.c pngerror.c \
 	pngget.c pngmem.c pngpread.c pngread.c pngrio.c pngrtran.c \
@@ -1096,8 +1096,10 @@ powerpc/powerpc_init.lo: powerpc/$(am__dirstamp) \
 powerpc/filter_vsx_intrinsics.lo: powerpc/$(am__dirstamp) \
 	powerpc/$(DEPDIR)/$(am__dirstamp)
 
-libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@.la: $(libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@_la_OBJECTS) $(libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@_la_DEPENDENCIES) $(EXTRA_libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@_la_DEPENDENCIES) 
+libpng.la: $(libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@_la_OBJECTS) $(libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@_la_DEPENDENCIES) $(EXTRA_libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@_la_DEPENDENCIES) 
 	$(AM_V_CCLD)$(libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@_la_LINK) -rpath $(libdir) $(libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@_la_OBJECTS) $(libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@_la_LIBADD) $(LIBS)
+libpng@PNGLIB_MAJOR@@PNGLIB_MINOR@.la: libpng.la
+	cp libpng.la $@
 loongarch/$(am__dirstamp):
 	@$(MKDIR_P) loongarch
 	@: >>loongarch/$(am__dirstamp)
@@ -2558,9 +2560,9 @@ uninstall-libpng-pc:
 install-library-links:
 	@set -x; cd '$(DESTDIR)$(libdir)';\
 	for ext in $(EXT_LIST); do\
-	   rm -f "libpng.$$ext";\
-           if test -f "$(PNGLIB_BASENAME).$$ext"; then\
-              $(LN_S) "$(PNGLIB_BASENAME).$$ext" "libpng.$$ext" || exit 1;\
+	   rm -f "$(PNGLIB_BASENAME).$$ext";\
+           if test -f "libpng.$$ext"; then\
+              $(LN_S) "libpng.$$ext" "$(PNGLIB_BASENAME).$$ext" || exit 1;\
            fi;\
 	done
 
