Create the common location for all m4 autotool build scripts, build-aux/m4.

Update .gitignore.

build-aux/m4/bitcoin_subdir_to_include.m4

@@ -0,0 +1,14 @@
+dnl BITCOIN_SUBDIR_TO_INCLUDE([CPPFLAGS-VARIABLE-NAME],[SUBDIRECTORY-NAME],[HEADER-FILE])
+dnl SUBDIRECTORY-NAME must end with a path separator
+AC_DEFUN([BITCOIN_SUBDIR_TO_INCLUDE],[
+  if test "x$2" = "x"; then
+    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`]
+    AC_MSG_RESULT([${newinclpath}])
+    if test "x${newinclpath}" != "x"; then
+      eval "$1=\"\$$1\"' -I${newinclpath}'"
+    fi
+  fi
+])