Bazel: Add optional support for GLPK.

Try it out:
bazel run --define USE_GLPK= cpp:linear_programming

examples/WORKSPACE

@@ -28,3 +28,11 @@ http_archive(
     strip_prefix = "protobuf-b4b0e304be5a68de3d0ee1af9b286f958750f5e4",
     sha256 = "ff771a662fb6bd4d3cc209bcccedef3e93980a49f71df1e987f6afa3bcdcba3a",
 )
+
+new_http_archive(
+    name = "glpk",
+    url = "http://ftp.gnu.org/gnu/glpk/glpk-4.47.tar.gz",
+    sha256 = "c35438e3ba74a8d85236810e6b84879272c87cfa7473b4075201e2967839f48d",
+    build_file = "../src/glpk.BUILD",
+)
+

src/WORKSPACE

@@ -30,4 +30,12 @@ new_http_archive(
     url = "https://github.com/google/googletest/archive/release-1.8.0.zip",
     build_file = "gtest.BUILD",
     strip_prefix = "googletest-release-1.8.0/googletest",
-)
+)
+
+new_http_archive(
+    name = "glpk",
+    url = "http://ftp.gnu.org/gnu/glpk/glpk-4.47.tar.gz",
+    sha256 = "c35438e3ba74a8d85236810e6b84879272c87cfa7473b4075201e2967839f48d",
+    build_file = "glpk.BUILD",
+)
+

src/glpk.BUILD

@@ -0,0 +1,20 @@
+cc_library(
+    name = "glpk",
+    srcs = glob([
+        "glpk-4.47/src/*.c",
+        "glpk-4.47/src/*/*.c",
+        "glpk-4.47/src/*.h",
+        "glpk-4.47/src/*/*.h",
+    ]),
+    hdrs = [
+        "glpk-4.47/src/glpk.h",
+    ],
+    copts = [
+        "-Wno-error",
+        "-w",
+        "-Iexternal/glpk/glpk-4.47/src",
+        "-DHAVE_ZLIB",
+    ],
+    includes=["glpk-4.47/src"],
+    visibility = ["//visibility:public"],
+)

src/linear_solver/BUILD

@@ -1,5 +1,12 @@
 package(default_visibility = ["//visibility:public"])
 
+# If you want to use the GLPK solver, build with '--define USE_GLPK=' (or add
+# it to your bazel.rc file). This will download, build and link to GLPK.
+config_setting(
+    name = "with_glpk",
+    values = { "define": "USE_GLPK=" }
+)
+
 proto_library(
     name = "linear_solver_proto",
     srcs = ["linear_solver.proto"],
@@ -20,7 +27,10 @@ cc_library(
         "linear_solver.cc",
         "model_exporter.cc",
         "model_validator.cc",
-    ],
+    ] +  select({
+        ":with_glpk": ["glpk_interface.cc"],
+        "//conditions:default": [],
+    }),
     hdrs = [
         "glop_utils.h",
         "linear_expr.h",
@@ -32,13 +42,21 @@ cc_library(
         "-DUSE_GLOP",
         "-DUSE_BOP",
     ],
+    defines = [
+        "HAVE_CONFIG_H"
+    ] +  select({
+        ":with_glpk": ["USE_GLPK"],
+        "//conditions:default": [],
+    }),
     visibility = ["//visibility:public"],
     deps = [
         ":linear_solver_cc_proto",
+        "//base:timer",
         "//base",
         "//base:file",
         "//base:hash",
         "//base:map_util",
+        "//base:stl_util",
         "//base:random",
         "//bop:bop_parameters_cc_proto",
         "//bop:integral_solver",
@@ -47,5 +65,9 @@ cc_library(
         "//lp_data",
         "//util:fp_utils",
         "//util:proto_tools",
-    ],
+    ] + select({
+        ":with_glpk": ["@glpk//:glpk"],
+        "//conditions:default": [],
+    }),
 )
+

src/linear_solver/linear_solver.h

@@ -149,7 +149,6 @@
 #include "base/integral_types.h"
 #include "base/logging.h"
 #include "base/timer.h"
-#include "glop/parameters.pb.h"
 #include "linear_solver/linear_expr.h"
 #include "linear_solver/linear_solver.pb.h"