libpar2 does not install headers when running `make install`

[Maarten]

When running `./configure && make && make install`, only the library + the config.h header are installed.
No header is copied.
Using the patch below, all headers are to `$(includedir)/libpar2`.
Additionally, an automake warning is fixed. (caused by automake 1.16.3)

```patch
--- Makefile.am
+++ Makefile.am
@@ -50,24 +50,42 @@
  verificationpacket.cpp verificationpacket.h

 LDADD = -lstdc++ $(SIGC_LIBS)
-INCLUDES = $(SIGC_CFLAGS)
+AM_CPPFLAGS = $(SIGC_CFLAGS)
 AM_CXXFLAGS = -Wall

 EXTRA_DIST = PORTING ROADMAP debian/changelog debian/compat debian/control \
  debian/copyright debian/libpar2-0-dev.dirs debian/docs \
- debian/rules debian/libpar2-0.dirs
-#par2cmdline.sln par2cmdline.vcproj \
-# testdata.tar.gz pretest test1 test2 test3 test4 test5 test6 \
-# posttest
-
-#TESTS = pretest test1 test2 test3 test4 test5 test6 posttest
+ debian/rules debian/libpar2-0.dirs

 library_includedir=$(includedir)/libpar2
-library_include_HEADERS = *.h
-
-
-
-libpar2_configdir = $(libdir)/libpar2/include
-libpar2_config_DATA = config.h
+library_include_HEADERS = \
+ commandline.h \
+ config.h \
+ crc.h \
+ creatorpacket.h \
+ criticalpacket.h \
+ datablock.h \
+ descriptionpacket.h \
+ diskfile.h \
+ filechecksummer.h \
+ galois.h \
+ letype.h \
+ libpar2.h \
+ mainpacket.h \
+ md5.h \
+ par1fileformat.h \
+ par1repairer.h \
+ par1repairersourcefile.h \
+ par2cmdline.h \
+ par2creator.h \
+ par2creatorsourcefile.h \
+ par2fileformat.h \
+ par2repairer.h \
+ par2repairersourcefile.h \
+ parheaders.h \
+ recoverypacket.h \
+ reedsolomon.h \
+ verificationhashtable.h \
+ verificationpacket.h

 libpar2_la_LDFLAGS = -version-info 1:0:0
--- configure.ac
+++ configure.ac
@@ -28,7 +28,7 @@

 AM_CONFIG_HEADER(config.h)
 AC_CONFIG_MACRO_DIR([m4])
-AM_INIT_AUTOMAKE(libpar2,0.3.1)
+AM_INIT_AUTOMAKE

 dnl Checks for programs.
```