Since boost 1.88, ::process::v1 is deprecated and ::process::v2 is the
default. AX_BOOST_PROCESS macro is failing to detect the library because
of the following compile failure :

> configure:24504: checking whether the Boost::Process library is available
> configure:24530: g++ -c   -I/usr/include conftest.cpp >&5
> conftest.cpp: In function 'int main()':
> conftest.cpp:59:17: error: 'child' is not a member of 'boost::process'
>    59 | boost::process::child* child = new boost::process::child; delete child;
>       |                 ^~~~~
> conftest.cpp:59:24: error: 'child' was not declared in this scope
>    59 | boost::process::child* child = new boost::process::child; delete child;
>       |                        ^~~~~
> conftest.cpp:59:52: error: 'child' in namespace 'boost::process' does not name a type
>    59 | boost::process::child* child = new boost::process::child; delete child;
>       |                                                    ^~~~~
> conftest.cpp:59:59: error: type '<type error>' argument given to 'delete', expected pointer
>    59 | boost::process::child* child = new boost::process::child; delete child;
>       |                                                           ^~~~~~~~~~~~
> configure:24530: $? = 1

1. boostorg/process@58586e4
2. boostorg/process@2ccd97c

Signed-off-by: Fabrice Delliaux <[email protected]>