build: optionally skip external warnings

Add an option to `./configure` to suppress compilation warnings from
external headers. The option is off by default (no change in behavior,
show warnings from external headers).

This option is useful if e.g. Boost or Qt is installed outside of
`/usr/include` (warnings from headers in `/usr/include` are already
suppressed by default) and those warnings stand in the way of compiling
Bitcoin Core with `-Werror[=...]` or they just clutter the build output
too much and make our own warnings hard to spot.

configure.ac

@@ -190,6 +190,16 @@ AC_ARG_ENABLE([ccache],
   [use_ccache=$enableval],
   [use_ccache=auto])
 
+dnl Suppress warnings from external headers (e.g. Boost, Qt).
+dnl May be useful if warnings from external headers clutter the build output
+dnl too much, so that it becomes difficult to spot Bitcoin Core warnings
+dnl or if they cause a build failure with --enable-werror.
+AC_ARG_ENABLE([suppress-external-warnings],
+  [AS_HELP_STRING([--enable-suppress-external-warnings],
+                  [Suppress warnings from external headers (default is no)])],
+  [suppress_external_warnings=$enableval],
+  [suppress_external_warnings=no])
+
 AC_ARG_ENABLE([lcov],
   [AS_HELP_STRING([--enable-lcov],
   [enable lcov testing (default is no)])],
@@ -1149,6 +1159,18 @@ AC_SUBST(LEVELDB_CPPFLAGS)
 AC_SUBST(LIBLEVELDB)
 AC_SUBST(LIBMEMENV)
 
+dnl SUPPRESSED_CPPFLAGS=SUPPRESS_WARNINGS([$SOME_CPPFLAGS])
+dnl Replace -I with -isystem in $SOME_CPPFLAGS to suppress warnings from
+dnl headers from its include directories and return the result.
+dnl See -isystem documentation:
+dnl https://gcc.gnu.org/onlinedocs/gcc/Directory-Options.html
+dnl https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-isystem-directory
+dnl Do not change "-I/usr/include" to "-isystem /usr/include" because that
+dnl is not necessary (/usr/include is already a system directory) and because
+dnl it would break GCC's #include_next.
+AC_DEFUN([SUPPRESS_WARNINGS],
+         [$(echo $1 |${SED} -E -e 's/(^| )-I/\1-isystem /g' -e 's;-isystem /usr/include([/ ]|$);-I/usr/include\1;g')])
+
 dnl enable-fuzz should disable all other targets
 if test "x$enable_fuzz" = "xyes"; then
   AC_MSG_WARN(enable-fuzz will disable all other targets)
@@ -1170,11 +1192,22 @@ else
 
   dnl sets $bitcoin_enable_qt, $bitcoin_enable_qt_test, $bitcoin_enable_qt_dbus
   BITCOIN_QT_CONFIGURE([5.5.1])
+
+  dnl Keep a copy of the original $QT_INCLUDES and use it when invoking qt's moc
+  QT_INCLUDES_UNSUPPRESSED=$QT_INCLUDES
+  if test x$suppress_external_warnings != xno ; then
+    QT_INCLUDES=SUPPRESS_WARNINGS($QT_INCLUDES)
+    QT_DBUS_INCLUDES=SUPPRESS_WARNINGS($QT_DBUS_INCLUDES)
+    QT_TEST_INCLUDES=SUPPRESS_WARNINGS($QT_TEST_INCLUDES)
+  fi
 fi
 
 if test x$enable_wallet != xno; then
     dnl Check for libdb_cxx only if wallet enabled
     BITCOIN_FIND_BDB48
+    if test x$suppress_external_warnings != xno ; then
+        BDB_CPPFLAGS=SUPPRESS_WARNINGS($BDB_CPPFLAGS)
+    fi
 fi
 
 dnl Check for libminiupnpc (optional)
@@ -1229,6 +1262,10 @@ AX_BOOST_THREAD
 dnl Opt-in to boost-process
 AS_IF([ test x$with_boost_process != x ], [ AX_BOOST_PROCESS ], [ ax_cv_boost_process=no ] )
 
+if test x$suppress_external_warnings != xno; then
+    BOOST_CPPFLAGS=SUPPRESS_WARNINGS($BOOST_CPPFLAGS)
+fi
+
 dnl Boost 1.56 through 1.62 allow using std::atomic instead of its own atomic
 dnl counter implementations. In 1.63 and later the std::atomic approach is default.
 m4_pattern_allow(DBOOST_AC_USE_STD_ATOMIC) dnl otherwise it's treated like a macro

src/Makefile.qt.include

@@ -379,11 +379,11 @@ ui_%.h: %.ui
 	$(AM_V_GEN) QT_SELECT=$(QT_SELECT) $(UIC) -o $@ $< || (echo "Error creating $@"; false)
 
 %.moc: %.cpp
-	$(AM_V_GEN) QT_SELECT=$(QT_SELECT) $(MOC) $(DEFAULT_INCLUDES) $(QT_INCLUDES) $(MOC_DEFS) $< | \
+	$(AM_V_GEN) QT_SELECT=$(QT_SELECT) $(MOC) $(DEFAULT_INCLUDES) $(QT_INCLUDES_UNSUPPRESSED) $(MOC_DEFS) $< | \
 	  $(SED) -e '/^\*\*.*Created:/d' -e '/^\*\*.*by:/d' > $@
 
 moc_%.cpp: %.h
-	$(AM_V_GEN) QT_SELECT=$(QT_SELECT) $(MOC) $(DEFAULT_INCLUDES) $(QT_INCLUDES) $(MOC_DEFS) $< | \
+	$(AM_V_GEN) QT_SELECT=$(QT_SELECT) $(MOC) $(DEFAULT_INCLUDES) $(QT_INCLUDES_UNSUPPRESSED) $(MOC_DEFS) $< | \
 	  $(SED) -e '/^\*\*.*Created:/d' -e '/^\*\*.*by:/d' > $@
 
 %.qm: %.ts