Fix warning: use AM_CPPFLAGS instead of INCLUDES #4650

Before the fix, 'autogen.sh' prints the below warning:

configure.ac:709: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')

Log of 'autogen.sh':

$ ./autogen.sh
libtoolize: putting auxiliary files in AC_CONFIG_AUX_DIR, `src/build-aux'.
libtoolize: copying file `src/build-aux/ltmain.sh'
libtoolize: putting macros in AC_CONFIG_MACRO_DIR, `src/m4'.
libtoolize: copying file `src/m4/libtool.m4'
libtoolize: copying file `src/m4/ltoptions.m4'
libtoolize: copying file `src/m4/ltsugar.m4'
libtoolize: copying file `src/m4/ltversion.m4'
libtoolize: copying file `src/m4/lt~obsolete.m4'
configure.ac:709: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')
configure.ac:709: warning: 'INCLUDES' is the old name for 'AM_CPPFLAGS' (or '*_CPPFLAGS')

This patch removes the warning.

Automatic sanity-testing: PASSED, see http://jenkins.bluematt.me/pull-tester/p4650_165e0a5dc87f4b7cc52f5441a071f69c4b36226f/ for binaries and test log. This test script verifies pulls every time they are updated. It, however, dies sometimes and fails to test properly. If you are waiting on a test, please check timestamps to verify that the test.log is moving at http://jenkins.bluematt.me/pull-tester/current/ Contact BlueMatt on freenode if something looks broken.

Maybe this is also related / can be removed now:

src/Makefile.am:AM_CPPFLAGS =  $(INCLUDES)

Some comments in src/m4 still refer to INCLUDES:

src/m4/bitcoin_qt.m4:dnl Requires: INCLUDES must be populated as necessary.
src/m4/bitcoin_qt.m4:dnl Requires: INCLUDES and LIBS must be populated as necessary.
src/m4/bitcoin_qt.m4:dnl Requires: INCLUDES and LIBS must be populated as necessary.

Mm, I'd rather eliminate the use of INCLUDES (now AM_CPPFLAGS) entirely. Since it was fairly straightforward, I went ahead and made the change. @kostaz Mind trying out https://github.com/theuni/bitcoin/commit/89d3cffffb28543e6bfa4aa59f4f33641190575f ?

It's a little wordy, but with the dependencies made explicit, moving the convenience libs to libtool libs (on my todo) will be simpler. It will also make things easier when we merge libsecp256k1, since we'll need some new logic to avoid roping in some libs.

@laanwj for the tests in the qt m4, those uses of INCLUDES and LIBS are necessary, as they have special meaning to the autoconf macros.

Yes, please.

@theuni can you submit that as a pull request? I too prefer separating out the specific dependencies instead of lobbing them in to INCLUDES / AM_CPPFLAGS.

Closing in favor of #4689