Add Bazel repositories for Flex and Bison. (#279)

This makes executable semantics build and pass tests for me without
installing either Bison or Flex. We just use the primitive toolchain
with the existing genrule as the packaged rules don't quite fit how
we're building and organizing the code.

Currently, this points at forks of the upstream rule repositories while
PRs I have sent there are going through, but this should be functional
for now and there doesn't seem to be any reason to wait for those PRs to
go through.

WORKSPACE

@@ -61,3 +61,57 @@ load("@llvm_bazel//:zlib.bzl", "llvm_zlib_system")
 
 # We require successful detection and use of a system zlib library.
 llvm_zlib_system(name = "llvm_zlib")
+
+# TODO(chandlerc): Replace this with an upstream release once the pull request
+# with our needed functionality lands:
+# https://github.com/jmillikin/rules_m4/pull/7
+#
+# Until then, this is pulling from that pull request's commit.
+http_archive(
+    name = "rules_m4",
+    strip_prefix = "rules_m4-add-extra-copts",
+    sha256 = "4d34917214e8890ad770bdf0c319c41c9201fffd770938b41a1d641d4b27e05c",
+    urls = ["https://github.com/chandlerc/rules_m4/archive/add-extra-copts.zip"],
+)
+
+load("@rules_m4//m4:m4.bzl", "m4_register_toolchains")
+
+# When building M4, disable all compiler warnings as we can't realistically fix
+# them anyways.
+m4_register_toolchains(extra_copts = ["-w"])
+
+# TODO(chandlerc): Replace this with an upstream release once the pull request
+# with our needed functionality lands:
+# https://github.com/jmillikin/rules_flex/pull/5
+#
+# Until then, this is pulling from that pull request's commit.
+http_archive(
+    name = "rules_flex",
+    strip_prefix = "rules_flex-add-extra-copts",
+    sha256 = "fd97c3ae23926507be1b95158a683cd41c628d201e852a325d38b5e9f821b752",
+    urls = ["https://github.com/chandlerc/rules_flex/archive/add-extra-copts.zip"],
+)
+
+load("@rules_flex//flex:flex.bzl", "flex_register_toolchains")
+
+# When building Flex, disable all compiler warnings as we can't realistically
+# fix them anyways.
+flex_register_toolchains(extra_copts = ["-w"])
+
+# TODO(chandlerc): Replace this with an upstream release once the pull request
+# with our needed functionality lands:
+# https://github.com/jmillikin/rules_bison/pull/7
+#
+# Until then, this is pulling from that pull request's commit.
+http_archive(
+    name = "rules_bison",
+    strip_prefix = "rules_bison-add-extra-copts",
+    sha256 = "c6e926f15214d903966dc950d759ec69116db67f148be114c119e4def0551eaa",
+    urls = ["https://github.com/chandlerc/rules_bison/archive/add-extra-copts.zip"],
+)
+
+load("@rules_bison//bison:bison.bzl", "bison_register_toolchains")
+
+# When building Bison, disable all compiler warnings as we can't realistically
+# fix them anyways.
+bison_register_toolchains(extra_copts = ["-w"])

executable_semantics/BUILD

@@ -44,19 +44,27 @@ genrule(
         "syntax.tab.cpp",
         "syntax.tab.h",
     ],
-    cmd = "bison " +
+    cmd = "M4=$(M4) $(BISON) " +
           "--output=$(location syntax.tab.cpp) " +
           "--defines=$(location syntax.tab.h) " +
           "$(location syntax.ypp)",
+    toolchains = [
+        "@rules_bison//bison:current_bison_toolchain",
+        "@rules_m4//m4:current_m4_toolchain",
+    ],
 )
 
 genrule(
     name = "syntax_flex_srcs",
     srcs = ["syntax.lpp"],
     outs = ["syntax.yy.cpp"],
-    cmd = "flex " +
+    cmd = "M4=$(M4) $(FLEX) " +
           "--outfile=$(location syntax.yy.cpp) " +
           "$(location syntax.lpp)",
+    toolchains = [
+        "@rules_flex//flex:current_flex_toolchain",
+        "@rules_m4//m4:current_m4_toolchain",
+    ],
 )
 
 EXAMPLES = [