configure: BITCOIN_SUBDIR_TO_INCLUDE: Improve compatibility with paths including space and multiline cpp output

Author: luke-jr

build-aux/m4/bitcoin_subdir_to_include.m4

@@ -9,7 +9,7 @@ AC_DEFUN([BITCOIN_SUBDIR_TO_INCLUDE],[
     AC_MSG_RESULT([default])
   else
     echo "#include <$2$3.h>" >conftest.cpp
-    newinclpath=`${CXXCPP} ${CPPFLAGS} -M conftest.cpp 2>/dev/null | [ tr -d '\\n\\r\\\\' | sed -e 's/^.*[[:space:]:]\(\/[^[:space:]]*\)]$3[\.h[[:space:]].*$/\1/' -e t -e d`]
+    newinclpath=$(${CXXCPP} ${CPPFLAGS} -M conftest.cpp 2>/dev/null | sed [-E -e ':a' -e '/\\$/!b b' -e N -e 's/\\\n/ /' -e 't a' -e ':b' -e 's/^[^:]*:[[:space:]]*(([^[:space:]\]|\\.)*[[:space:]])*(([^[:space:]\]|\\.)*)]$3\.h[([[:space:]].*)?$/\3/' -e 't' -e d])
     AC_MSG_RESULT([${newinclpath}])
     if test "${newinclpath}" != ""; then
       eval "$1=\"\$$1\"' -I${newinclpath}'"