Merge bitcoin#25770: build: Fix help string for --enable-external-signer configure option

8df063e build: Fix help string for `--enable-external-signer` configure option (Hennadii Stepanov)

Pull request description:

  This PR is a follow up of bitcoin#24065 and fixes the help string according to the actual default value https://github.com/bitcoin/bitcoin/blob/816ca01650f4cc66a61ac2f9b0f8b74cd9cd0cf8/configure.ac#L324-L327

ACKs for top commit:
  kristapsk:
    cr utACK 8df063e
  jarolrod:
    ACK 8df063e

Tree-SHA512: ad3f457a53c9238ddd8ded9efd1224e564e6cb9da8b7ff7733a11e32a7daad5c0f6c6223509218f44944a874470cb0d2447897662eaf4e78c763b30785717c50

Author: fanquake

configure.ac

@@ -332,7 +332,7 @@ AC_ARG_ENABLE([werror],
     [enable_werror=no])
 
 AC_ARG_ENABLE([external-signer],
-    [AS_HELP_STRING([--enable-external-signer],[compile external signer support (default is yes, requires Boost::Process)])],
+    [AS_HELP_STRING([--enable-external-signer],[compile external signer support (default is auto, requires Boost::Process)])],
     [use_external_signer=$enableval],
     [use_external_signer=auto])