[devel] Don't include standard header files in png.h while building the symbol

table, to avoid cpp failure on SunOS (introduced PNG_BUILDING_SYMBOL_TABLE
macro).

Makefile.am

diff --git a/Makefile.am b/Makefile.am
index 18f87b4..7b8c494 100644
--- a/Makefile.am
+++ b/Makefile.am
@@ -124,7 +124,7 @@
 SYMBOL_CFLAGS = -DPNGLIB_LIBNAME='PNG@PNGLIB_MAJOR@@PNGLIB_MINOR@_0'\
 		-DPNGLIB_VERSION='@PNGLIB_VERSION@'\
 		-DSYMBOL_PREFIX='$(SYMBOL_PREFIX)'\
-		-DPNG_NO_USE_READ_MACROS
+		-DPNG_NO_USE_READ_MACROS -DPNG_BUILDING_SYMBOL_TABLE
 
 .dfn.out:
 	rm -f $@ dfn.c dfn?.out