configure: add --enable-external-signer

This option replaces --with-boost-process

This prepares external signer support to be disabled by default.
It adds a configure option to enable this feature and to check
if Boost::Process is present.

This also exposes ENABLE_EXTERNAL_SIGNER to the test suite via test/config.ini

doc/Doxyfile.in

@@ -2073,7 +2073,7 @@ INCLUDE_FILE_PATTERNS  =
 # recursively expanded use the := operator instead of the = operator.
 # This tag requires that the tag ENABLE_PREPROCESSING is set to YES.
 
-PREDEFINED             = HAVE_BOOST_PROCESS
+PREDEFINED             = ENABLE_EXTERNAL_SIGNER
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then this
 # tag can be used to specify a list of macro names that should be expanded. The