Build system: Fix failure to correctly link to zlib (#32)

Checking for the header is NOT sufficient when utilizing its shared
library symbols. Look it up with pkg-config explicitly, and explicitly
add it to ensure that at runtime, libppd has its own DT_NEEDED
dependency on libz.so; if libppd successfully links at all -- not a
given, if -Wl,-no-undefined is used -- then it *may* transitively get
libz.so from its recursive dependencies, but this is no guarantee at
all.

Fixes failure to build discovered at https://bugs.gentoo.org/920273

Makefile.am

@@ -120,9 +120,11 @@ libppd_la_SOURCES = \
 	$(pkgppddefs_DATA)
 libppd_la_LIBADD = \
 	$(LIBCUPSFILTERS_LIBS) \
+	$(ZLIB_LIBS) \
 	$(CUPS_LIBS)
 libppd_la_CFLAGS = \
 	$(LIBCUPSFILTERS_CFLAGS) \
+	$(ZLIB_CFLAGS) \
 	$(CUPS_CFLAGS)
 libppd_la_CXXFLAGS = \
 	$(libppd_la_CFLAGS)

configure.ac

@@ -164,6 +164,13 @@ AC_SUBST(CUPS_STATEDIR)
 # ========================
 PKG_CHECK_MODULES([LIBCUPSFILTERS], [libcupsfilters])
 
+# ==============
+# Check for zlib
+# ==============
+PKG_CHECK_MODULES([ZLIB], [zlib], ,[
+        AC_CHECK_HEADER([zlib.h])
+        AC_CHECK_LIB([z], [deflateInit])])
+
 # ============================================================
 # Check for whether we want to install the testppdfile utility
 # ============================================================
@@ -220,7 +227,6 @@ AC_CHECK_HEADERS([stdlib.h])
 AC_CHECK_HEADERS([sys/stat.h])
 AC_CHECK_HEADERS([sys/types.h])
 AC_CHECK_HEADERS([unistd.h])
-AC_CHECK_HEADERS([zlib.h])
 AC_CHECK_HEADERS([endian.h])
 AC_CHECK_HEADERS([dirent.h])
 AC_CHECK_HEADERS([sys/ioctl.h])