Undefined reference to `scoped_static_mutex_lock` on Windows R 3.6

I don't know if it will want changes, but I ran into this issue when trying to compile this on Windows and R 3.6 specifically:

Rcpp::sourceCpp(code = '
  // [[Rcpp::depends(BH)]]
  #include <Rcpp.h>
  #include <boost/regex.hpp>

  // [[Rcpp::export]]
  bool startswith_a(std::string term) {
    boost::regex a("^a");
    return boost::regex_match(term, a);
  }
')

C:/Rtools/mingw_64/bin/g++ -std=gnu++11  -I"C:/PROGRA~1/R/R-36~1.3/include" -DNDEBUG   -I"C:/Users/Username/Documents/R/win-library/3.6/Rcpp/include" -I"C:/Users/Username/Documents/R/win-library/3.6/BH/include" -I"C:/Users/Username/AppData/Local/Temp/RtmpIzbuqE/sourceCpp-x86_64-w64-mingw32-1.0.11"        -O2 -Wall  -mtune=core2 -c file7894567d42f4.cpp -o file7894567d42f4.o
C:/Rtools/mingw_64/bin/g++ -std=gnu++11 -shared -s -static-libgcc -o sourceCpp_103.dll tmp.def file7894567d42f4.o -LC:/PROGRA~1/R/R-36~1.3/bin/x64 -lR
file7894567d42f4.o:file7894567d42f4.cpp:(.text$_ZN5boost16cpp_regex_traitsIcE16get_catalog_nameEv[_ZN5boost16cpp_regex_traitsIcE16get_catalog_nameEv]+0x1f): undefined reference to `boost::scoped_static_mutex_lock::scoped_static_mutex_lock(boost::static_mutex&, bool)'
file7894567d42f4.o:file7894567d42f4.cpp:(.text$_ZN5boost16cpp_regex_traitsIcE16get_catalog_nameEv[_ZN5boost16cpp_regex_traitsIcE16get_catalog_nameEv]+0x3f): undefined reference to `boost::scoped_static_mutex_lock::~scoped_static_mutex_lock()'
file7894567d42f4.o:file7894567d42f4.cpp:(.text$_ZN5boost16cpp_regex_traitsIcE16get_catalog_nameEv[_ZN5boost16cpp_regex_traitsIcE16get_catalog_nameEv]+0xa2): undefined reference to `boost::scoped_static_mutex_lock::~scoped_static_mutex_lock()'
file7894567d42f4.o:file7894567d42f4.cpp:(.text$_ZN5boost12object_cacheINS_13re_detail_50021cpp_regex_traits_baseIcEENS1_31cpp_regex_traits_implementationIcEEE3getERKS3_y[_ZN5boost12object_cacheINS_13re_detail_50021cpp_regex_traits_baseIcEENS1_31cpp_regex_traits_implementationIcEEE3getERKS3_y]+0x27): undefined reference to `boost::scoped_static_mutex_lock::scoped_static_mutex_lock(boost::static_mutex&, bool)'
file7894567d42f4.o:file7894567d42f4.cpp:(.text$_ZN5boost12object_cacheINS_13re_detail_50021cpp_regex_traits_baseIcEENS1_31cpp_regex_traits_implementationIcEEE3getERKS3_y[_ZN5boost12object_cacheINS_13re_detail_50021cpp_regex_traits_baseIcEENS1_31cpp_regex_traits_implementationIcEEE3getERKS3_y]+0x44): undefined reference to `boost::scoped_static_mutex_lock::~scoped_static_mutex_lock()'
file7894567d42f4.o:file7894567d42f4.cpp:(.text$_ZN5boost12object_cacheINS_13re_detail_50021cpp_regex_traits_baseIcEENS1_31cpp_regex_traits_implementationIcEEE3getERKS3_y[_ZN5boost12object_cacheINS_13re_detail_50021cpp_regex_traits_baseIcEENS1_31cpp_regex_traits_implementationIcEEE3getERKS3_y]+0xa7): undefined reference to `boost::scoped_static_mutex_lock::~scoped_static_mutex_lock()'
collect2.exe: error: ld returned 1 exit status

But that works when defining either BOOST_REGEX_STANDALONE or BOOST_REGEX_USE_C_LOCALE (as far as I tested) beforehand:

Rcpp::sourceCpp(code = '
  // [[Rcpp::depends(BH)]]
  #include <Rcpp.h>
  #define BOOST_REGEX_USE_C_LOCALE
  #include <boost/regex.hpp>

  // [[Rcpp::export]]
  bool startswith_a(std::string term) {
    boost::regex a("^a");
    return boost::regex_match(term, a);
  }
')

I'm not sure why those work or what difference they might make, so, if defining those is an acceptable solution, I wonder:

Is one of those (or another option) preferable? I went with BOOST_REGEX_USE_C_LOCALE for now because it seemed like it would have fewer side effects.
Would it be better to only define one of those when necessary (since it works without them on other platforms and newer versions of R)? If so, what would a good check for that be?

eddelbuettel / bh

Undefined reference to `scoped_static_mutex_lock` on Windows R 3.6 #93