Fix *.d parsing with gcc 9.3.0

Before this commit:

  $ rake
  ...
  $ rm build/host/src/gc.o
  $ rake
  rake aborted!
  Don't know how to build task '/home/foo/mruby/build/host/src/gc.o' (See the list of available tasks with `rake --tasks`)
  ...

After this commit:

  $ rake
  ...
  $ rm build/host/src/gc.o
  $ rake
  CC    src/gc.c -> build/host/src/gc.o
  ...

With gcc 9.3.0 on Ubuntu 20.04, build/host/src/array.d looks like:

  /build/host/src/array.o: /src/array.c \
   /include/mruby.h /include/mrbconf.h \
   /include/mruby/common.h \
   ...
   /include/mruby/range.h \
   /src/value_array.h

and it has been parsed to:

  [
                                           # /src/array.c missing
    "/include/mruby.h /include/mrbconf.h", # incorrectly parsed
    "/include/mruby/common.h",
    ...
    "/src/value_array.h",
  ]

After this change, *.d will be parsed correctly.

  [
    "/src/array.c",
    "/include/mruby.h",
    "/include/mrbconf.h",
    "/include/mruby/common.h",
    ...
    "/src/value_array.h",
  ]

lib/mruby/build/command.rb

@@ -139,6 +139,15 @@ module MRuby
     #     /include/mruby/value.h \
     #     /src/value_array.h
     #
+    # ==== Without <tt>-MP</tt> compiler flag (gcc (Ubuntu 9.3.0-10ubuntu2) 9.3.0)
+    #
+    #   /build/host/src/array.o: /src/array.c \
+    #    /include/mruby.h /include/mrbconf.h \
+    #    /include/mruby/common.h \
+    #    ...
+    #    /include/mruby/range.h \
+    #    /src/value_array.h
+    #
     # ==== With <tt>-MP</tt> compiler flag
     #
     #   /build/host/src/array.o: \
@@ -155,11 +164,18 @@ module MRuby
     #
     def get_dependencies(file)
       file = file.ext('d') unless File.extname(file) == '.d'
-      deps = []
-      if File.exist?(file)
-        File.foreach(file){|line| deps << $1 if /^ +(.*?)(?: *\\)?$/ =~ line}
-        deps.uniq!
-      end
+      return [MRUBY_CONFIG] unless File.exist?(file)
+
+      deps = "".gsub("\\\n ", "").split("\n").map do |dep_line|
+        # dep_line:
+        # - "/build/host/src/array.o:   /src/array.c   /include/mruby/common.h ..."
+        # - ""
+        # - "/include/mruby/common.h:"
+        dep_line.scan(/^\S+:\s+(.+)$/).flatten.map { |s| s.split(' ') }.flatten
+        # => ["/src/array.c", "/include/mruby/common.h" , ...]
+        #    []
+        #    []
+      end.flatten.uniq
       deps << MRUBY_CONFIG
     end
   end