Check whether C++ compiler works. #5195

The macro AC_PROG_CXX doesn't really check whether the C++ compiler works (compare with AC_PROG_CC). This change adds a new local macro AC_PROG_CXX_WORKS that is really checking whether C++ compiler works. See #5177. If there is no C++ compiler installed, configure prints

checking whether the C++ compiler works... no configure: error: C++ compiler doesn't work.

concept ACK

It looks like libtool is the culprit here. Going by the docs and the linked discussion from #5177, checking for c++ before c (which libtool does) is the suggested fix. Something like:

diff --git a/configure.ac b/configure.ac
index 17efd98..3ccec09 100644
--- a/configure.ac
+++ b/configure.ac
@@ -24,6 +24,13 @@ case $host in
   ;;
 esac

+if test "x${CXXFLAGS+set}" = "xset"; then
+  CXXFLAGS_overridden=yes
+else
+  CXXFLAGS_overridden=no
+fi
+
+AC_PROG_CXX
 LT_INIT([disable-shared])

 AH_TOP([#ifndef BITCOIN_CONFIG_H])
@@ -40,12 +47,6 @@ m4_include([pkg.m4])
 dnl faketime breaks configure and is only needed for make. Disable it here.
 unset FAKETIME

-if test "x${CXXFLAGS+set}" = "xset"; then
-  CXXFLAGS_overridden=yes
-else
-  CXXFLAGS_overridden=no
-fi
-
 dnl ==============================================================
 dnl Setup for automake
 dnl ==============================================================
@@ -60,7 +61,6 @@ dnl make the compilation flags quiet unless V=1 is used
 m4_ifdef([AM_SILENT_RULES], [AM_SILENT_RULES([yes])])

 dnl Checks for programs.
-AC_PROG_CXX
 AC_PROG_CC
 AC_PROG_CPP
 AC_PROG_CXXCPP

Yup, this works. It is much better fix then the proposed one. Doesn't bring new autoconfisms, only moves them around!

@paveljanik are you going to update this pull to that?

@wumpus: yes.

Pavel Janík

My #5214 should address this also. Apologies for coming in over the top but had this 'tidy up' sitting around for a while ...

Jebus, people, this Wumpus IS NOT THE WUMPUS YOU ARE TALKING TO.

@wumpus: I'd like to apologize to you. I promise I'll use github's features more carefully next time :+1:

Closing, as overlaps with #5214