Merge bitcoin#18750: build: optionally skip external warnings

ba8950e build: optionally skip external warnings (Vasil Dimov)

Pull request description:

  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.

  `-isystem /usr/include` bricks GCC's `#include_next`, so we use
  `-idirafter` instead. This way we don't have to treat `/usr/include`
  specially.

ACKs for top commit:
  practicalswift:
    ACK ba8950e: diff looks correct!
  hebasto:
    ACK ba8950e, tested on Linux Mint 20 (x86_64).
  luke-jr:
    utACK ba8950e

Tree-SHA512: 9b54fae8590be6c79f2688a5aca09e0a9067f481dabecdd49bb278c08a62ac2b0cc704c894fbd53240e77ac84da0c7a237845df0a696cfbdb0359e1c8e2e10c9

configure.ac

@@ -172,6 +172,16 @@ 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)])],
@@ -1077,9 +1087,24 @@ 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([/ ]|$);-I/usr/include\1;g')])
+
 if test x$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)
@@ -1116,6 +1141,14 @@ BITCOIN_QT_INIT
 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
+
 if test x$build_bitcoin_utils$build_bitcoind$bitcoin_enable_qt$use_tests$use_bench = xnonononono; then
     use_boost=no
 else
@@ -1137,6 +1170,10 @@ AX_BOOST_FILESYSTEM
 AX_BOOST_THREAD
 AX_BOOST_CHRONO
 
+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

@@ -490,11 +490,11 @@ 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