configure: fix for --enable-pthreads

Better handle options conflicts that can occur if --enable-pthreads.

Bug: https://github.com/curl/curl/pull/1295
Reported-by: Marc-Antoine Perennou

configure.ac

@@ -3427,8 +3427,28 @@ AC_HELP_STRING([--disable-pthreads],[Disable POSIX threads]),
        want_pthreads=auto
        ]
 )
-if test "$want_thres" = "yes" && test "$dontwant_rt" = "no" && \
-   test "$want_pthreads" != "no"; then
+
+dnl turn off pthreads if rt is disabled
+if test "$want_pthreads" != "no"; then
+  if test "$want_pthreads" = "yes" && test "$dontwant_rt" = "yes"; then
+    AC_MSG_ERROR([options --enable-pthreads and --disable-rt are mutually exclusive])
+  fi
+  if test "$dontwant_rt" != "no"; then
+    dnl if --enable-pthreads was explicit then warn it's being ignored
+    if test "$want_pthreads" = "yes"; then
+      AC_MSG_WARN([--enable-pthreads Ignored since librt is disabled.])
+    fi
+    want_pthreads=no
+  fi
+fi
+
+dnl turn off pthreads if no threaded resolver
+if test "$want_pthreads" != "no" && test "$want_thres" != "yes"; then
+  want_pthreads=no
+fi
+
+dnl detect pthreads
+if test "$want_pthreads" != "no"; then
   AC_CHECK_HEADER(pthread.h,
     [ AC_DEFINE(HAVE_PTHREAD_H, 1, [if you have <pthread.h>])
       save_CFLAGS="$CFLAGS"
@@ -3452,11 +3472,12 @@ if test "$want_thres" = "yes" && test "$dontwant_rt" = "no" && \
       fi
   ])
 fi
-if test "x$USE_THREADS_POSIX" != "x1"; then
+
+dnl threaded resolver check
+if test "$want_thres" = "yes" && test "x$USE_THREADS_POSIX" != "x1"; then
   if test "$want_pthreads" = "yes"; then
     AC_MSG_ERROR([--enable-pthreads but pthreads was not found])
   fi
-  if test "$want_thres" = "yes"; then
   dnl If native Windows fallback on Win32 threads since no POSIX threads
   if test "$curl_cv_native_windows" = "yes"; then
     USE_THREADS_WIN32=1
@@ -3465,10 +3486,8 @@ if test "x$USE_THREADS_POSIX" != "x1"; then
   else
     AC_MSG_ERROR([Threaded resolver enabled but no thread library found])
   fi
-  fi
 fi
 
-
 dnl ************************************************************
 dnl disable verbose text strings
 dnl

m4/curl-confopts.m4

@@ -346,10 +346,6 @@ AC_DEFUN([CURL_CHECK_OPTION_RT], [
       AC_MSG_RESULT([no])
       ;;
   esac
-  dnl TODO: may require mutual exclusion
-  if test "$dontwant_rt" = "yes" && test "$want_thres" = "yes" ; then
-    AC_MSG_ERROR([options --disable-rt and --enable-thread-resolver are mutually exclusive, at most one can be selected.])
-  fi
 ])