Fix FreeBSD errors when using tasks.

FreeBSD gcc, instead of failing at link time when using multithreading
and forgetting add the -pthread option it throws an exception at
runtime. This change default to using the -pthread option for bootstrap
on all platforms. It also turns on threading=multi on the jam build.

.ci/azp-linux-asan-test.yml

@@ -15,7 +15,7 @@ steps:
   displayName: Build
 - bash: |
     set -e
-    export B2=`ls -1 ${PWD}/.build/*/debug/address-sanitizer-on/cxxstd-11-iso/b2`
+    export B2=`ls -1 ${PWD}/.build/*/debug/address-sanitizer-on/cxxstd-11-iso/threading-multi/b2`
     cd test
     ./test_all.py ${TOOLSET}
     cd ..

Jamroot.jam

@@ -27,6 +27,7 @@ project b2
     : build-dir .build
     : requirements
         <cxxstd>11
+        <threading>multi
         <toolset>msvc:<define>_CRT_SECURE_NO_WARNINGS=1
         <toolset>msvc:<define>_CRT_NONSTDC_NO_DEPRECATE=1
         <variant>debug:<define>B2_DEBUG=1

src/engine/build.sh

@@ -156,11 +156,11 @@ check_toolset ()
 
     # Prefer Clang (clang) on macOS..
     if test_toolset clang && test_uname Darwin && test_compiler clang++$TOOLSET_SUFFIX -x c++ -std=c++11 ; then B2_TOOLSET=clang$TOOLSET_SUFFIX ; return ${TRUE} ; fi
+    # GCC (gcc) with -pthread arg (for AIX and others)..
+    if test_toolset gcc && test_compiler g++$TOOLSET_SUFFIX -x c++ -std=c++11 -pthread ; then B2_TOOLSET=gcc$TOOLSET_SUFFIX ; return ${TRUE} ; fi
     # GCC (gcc)..
     if test_toolset gcc && test_compiler g++$TOOLSET_SUFFIX -x c++ -std=c++11 ; then B2_TOOLSET=gcc$TOOLSET_SUFFIX ; return ${TRUE} ; fi
     if test_toolset gcc && test_compiler g++$TOOLSET_SUFFIX -x c++ -std=c++11 -D_GNU_SOURCE ; then B2_TOOLSET=gcc$TOOLSET_SUFFIX ; return ${TRUE} ; fi
-    # GCC (gcc) with -pthread arg (for AIX and others)..
-    if test_toolset gcc && test_compiler g++$TOOLSET_SUFFIX -x c++ -std=c++11 -pthread ; then B2_TOOLSET=gcc$TOOLSET_SUFFIX ; return ${TRUE} ; fi
     # Clang (clang)..
     if test_toolset clang && test_compiler clang++$TOOLSET_SUFFIX -x c++ -std=c++11 ; then B2_TOOLSET=clang$TOOLSET_SUFFIX ; return ${TRUE} ; fi
     # Clang (clang) with -pthread arg (for FreeBSD and others)..

src/engine/tasks.cpp

@@ -319,7 +319,7 @@ unsigned get_parallelism(int parallelism)
 #if B2_USE_STD_THREADS
 	return parallelism >= 0
 		? parallelism
-		: std::min(unsigned(globs.jobs), system_info().cpu_thread_count()) - 1;
+		: std::min(unsigned(globs.jobs), system_info().cpu_thread_count());
 #else
 	return 0;
 #endif