[poppler] minor fix for CFLAGS about PNG headers

[mpsuzuki at hiroshima-u.ac.jp]

Hi,

During the cross build for MinGW environment, I found
that small minor fixes for PNG headers.

If PNG library is available, goo/PNGWriter.h includes
it (goo/Makefile.am appropriately reflect LIBPNG_CFLAGS).
But when SplashBitmap.cc (including PNGWriter.h) is
being compiled, LIBPNG_CFLAGS is not considered.
Thus, when PNG library is installed in irregular directory,
SplashBitmap.cc cannot be proprocessed correctly.
LIBPNG_CFLAGS should be added to INCLUDES by fixing
splash/Makefile.am.

Exactly same problem (PNGWriter.h) is found in
HtmlOutputDev.cc compilation, so utils/Makefile.am
should be improved.

I guess similar things are expected for JPEG and TIFF
libraries, but now LIBJPEG_CFLAGS, LIBTIFF_CFLAGS are
not handled by configure. Should I prepare some hooks
for them?

diff --git a/splash/Makefile.am b/splash/Makefile.am
index 914b308..afd31df 100644
--- a/splash/Makefile.am
+++ b/splash/Makefile.am
@@ -3,6 +3,10 @@ INCLUDES =					\
 	-I$(top_srcdir)/goo			\
 	$(FREETYPE_CFLAGS)
 
+if BUILD_LIBPNG
+INCLUDES += $(LIBPNG_CFLAGS)
+endif
+
 noinst_LTLIBRARIES = libsplash.la
 
 if ENABLE_XPDF_HEADERS
diff --git a/utils/Makefile.am b/utils/Makefile.am
index c0105d9..ad845c1 100644
--- a/utils/Makefile.am
+++ b/utils/Makefile.am
@@ -117,6 +117,12 @@ pdftohtml_SOURCES =				\
 	HtmlUtils.h				\
 	$(common)
 
+# HtmlOutputDev uses goo/PNGWriter.h that may depend on libpng header
+pdftohtml_CXXFLAGS = $(AM_CXXFLAGS)
+if BUILD_LIBPNG
+pdftohtml_CXXFLAGS += $(LIBPNG_CFLAGS)
+endif
+
 pdfseparate_SOURCES =				\
 	pdfseparate.cc				\
 	$(common)