Try to use pkg-config to find external dependencies (fixes qpdf#324)

ChangeLog

@@ -1,5 +1,8 @@
 2019-06-22  Jay Berkenbilt  <[email protected]>
 
+	* If pkg-config is available, use it to local libjpeg and zlib. If
+	not, fall back to old behavior. Fixes #324.
+
 	* The "make install" target explicitly sets a mode rather than
 	relying the user's umask. Fixes #326.
 

README-maintainer

@@ -121,6 +121,7 @@ RELEASE PREPARATION
     ```
     cp /usr/share/automake-1.11/install-sh .
     cp /usr/share/automake-1.11/mkinstalldirs .
+    cp /usr/share/aclocal/pkg.m4 m4
     ```
 
   The entire contents of the `m4` directory came from `libtool.m4`. If

aclocal.m4

@@ -18,3 +18,4 @@ m4_include([m4/ltoptions.m4])
 m4_include([m4/ltsugar.m4])
 m4_include([m4/ltversion.m4])
 m4_include([m4/lt~obsolete.m4])
+m4_include([m4/pkg.m4])

autofiles.sums

@@ -1,7 +1,10 @@
-5b50c329677bcc8c1d5a8b3654b79ccca8bbe80a452cc4bb14938370f316327a  configure.ac
+2784dbb5657cdd1ce359729771d8ad768becc3751dfac2b13b3eb11d146684a7  configure.ac
+9c264dc3fac489fd0178d3fa9fd29d5b0ccd37d561e926ef449fa01d99abded7  aclocal.m4
+79ee40c3867f4162a847c005a73a5e388e3882f71dd185d39d9ddf0312133086  libqpdf/qpdf/qpdf-config.h.in
 35bc5c645dc42d47f2daeea06f8f3e767c8a1aee6a35eb2b4854fd2ce66c3413  m4/ax_random_device.m4
 37f8897d5f68d7d484e5457832a8f190ddb7507fa2a467cb7ee2be40a4364643  m4/libtool.m4
 e77ebba8361b36f14b4d0927173a034b98c5d05049697a9ded84d85eb99a7990  m4/ltoptions.m4
 5a6735cda60e0ba0d1b706c0ef648f5d096298da46daefd9cdecdb6a0f4584d3  m4/ltsugar.m4
 a27b754709de61575197bf5a980696c98ae49da3f92f0de8ee7f42dd543b7465  m4/ltversion.m4
 26fa3285c35dd6ab00ed1e466ba92a17e4655e01897731ec18a587a4cf5e4f8d  m4/lt~obsolete.m4
+9fab676fae13feb97d5183a8ed86ae9398c76d21927c28bc59460d230f3e0884  m4/pkg.m4

autogen.sh

@@ -3,4 +3,4 @@ set -e
 aclocal -I m4
 autoheader
 autoconf
-sha256sum configure.ac m4/* >| autofiles.sums
+sha256sum configure.ac aclocal.m4 libqpdf/qpdf/qpdf-config.h.in m4/* >| autofiles.sums