build: boost::process detection doesn't pass include directory

[laanwj]

boost::process detection fails here (FreeBSD 13.0) with:

configure:32183: checking for Boost Process
configure:32196: c++ -std=c++17 -c -g -O2  -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS -DPROVIDE_FUZZ_MAIN_FUNCTION conftest.cpp >&5
conftest.cpp:80:10: fatal error: 'boost/process.hpp' file not found
#include <boost/process.hpp>
         ^~~~~~~~~~~~~~~~~~~
1 error generated.
configure:32196: $? = 1

The header in question does exist in /usr/local/include/boost/process.hpp. However, no include path is passed?

For comparison, the preceding boost::filesystem check, which passes -I/usr/local/include succeeds:

configure:32026: checking for exit in -lboost_filesystem
configure:32051: c++ -std=c++17 -o conftest -g -O2  -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS -DPROVIDE_FUZZ_MAIN_FUNCTION -I/usr/local/include  -L/usr/local/lib conftest.cpp -lboost_filesystem   -lboost_system >&5
configure:32051: $? = 0
configure:32061: result: yes

[theStack]

I can confirm the same behaviour on OpenBSD 6.9:

configure:31594: checking for Boost Process
configure:31607: c++ -std=c++17 -c -g -O2  -DHAVE_BUILD_INFO -D__STDC_FORMAT_MACROS -DPROVIDE_FUZZ_MAIN_FUNCTION conftest.cpp >&5
conftest.cpp:79:10: fatal error: 'boost/process.hpp' file not found
#include <boost/process.hpp>
         ^~~~~~~~~~~~~~~~~~~
1 error generated.

$ find / -name process.hpp
...
/usr/local/include/boost/process.hpp
...

[jarolrod]

Confirming the same behavior on my FreeBSD 13 machine; when trying to build off master:

checking for Boost Process... configure: error: Boost::Process is required for external signer support, but not available!