Merge bitcoin/bitcoin#25599: build: Check for std::atomic::exchange rather than std::atomic_exchange

4de4221 build: Check for std::atomic::exchange rather than std::atomic_exchange (Andrew Chow)

Pull request description:

  Our usage of std::atomic is with it's own exchange function, not std::atomic_exchange. So we should be looking specifically for that function.

  This removes the need for -latomic for riscv builds, which resolves a guix cross architecture reproducibility issue.

ACKs for top commit:
  hebasto:
    ACK 4de4221
  fanquake:
    ACK 4de4221

Tree-SHA512: dd8225fc9c6a335601f611700003d0249b9ef941efa502db39306129677929d013048e9221be1d6d7f0ea2d90313d4b87de239f441be21b25bea40a6c19a031e

Author: fanquake

build-aux/m4/l_atomic.m4

@@ -18,7 +18,7 @@ m4_define([_CHECK_ATOMIC_testbody], [[
 
   int main() {
     std::atomic<bool> lock{true};
-    std::atomic_exchange(&lock, false);
+    lock.exchange(false);
 
     std::atomic<std::chrono::seconds> t{0s};
     t.store(2s);
@@ -34,6 +34,8 @@ m4_define([_CHECK_ATOMIC_testbody], [[
 AC_DEFUN([CHECK_ATOMIC], [
 
   AC_LANG_PUSH(C++)
+  TEMP_CXXFLAGS="$CXXFLAGS"
+  CXXFLAGS="$CXXFLAGS $PTHREAD_CFLAGS"
 
   AC_MSG_CHECKING([whether std::atomic can be used without link library])
 
@@ -51,5 +53,6 @@ AC_DEFUN([CHECK_ATOMIC], [
         ])
     ])
 
+  CXXFLAGS="$TEMP_CXXFLAGS"
   AC_LANG_POP
 ])

configure.ac

@@ -87,9 +87,6 @@ else
 AX_CXX_COMPILE_STDCXX([20], [noext], [mandatory])
 fi
 
-dnl Check if -latomic is required for <std::atomic>
-CHECK_ATOMIC
-
 dnl check if additional link flags are required for std::filesystem
 CHECK_FILESYSTEM
 
@@ -887,6 +884,9 @@ AC_C_BIGENDIAN
 dnl Check for pthread compile/link requirements
 AX_PTHREAD
 
+dnl Check if -latomic is required for <std::atomic>
+CHECK_ATOMIC
+
 dnl The following macro will add the necessary defines to bitcoin-config.h, but
 dnl they also need to be passed down to any subprojects. Pull the results out of
 dnl the cache and add them to CPPFLAGS.