Added submodule for cimplot and some build code

This points to aloucks' fork instead of imgui-rs directly
so I can try out the PR. Doesn't seem to work as expected
yet though, trying to read the env var doesn't show anything.

I'm probably doing it wrong.

.gitmodules

@@ -0,0 +1,3 @@
+[submodule "implot-sys/third-party/cimplot"]
+	path = implot-sys/third-party/cimplot
+	url = https://github.com/cimgui/cimplot

implot-sys/Cargo.toml

@@ -10,3 +10,4 @@ build = "build.rs"
 
 [build-dependencies]
 cc = "1.0"
+imgui = { git = "https://github.com/aloucks/imgui-rs/", branch = "export_include" }

implot-sys/build.rs

@@ -1,6 +1,49 @@
+#![allow(dead_code)]
+
+// This is taken pretty vanilla from
+// https://github.com/Gekkio/imgui-rs/blob/master/imgui-sys/build.rs
+// for now, but expected to diverge from that over time.
+use std::fs;
 use std::io;
 
+const CPP_FILES: [&str; 2] = [
+    "third-party/cimplot/cimplot.cpp",
+    "third-party/cimplot/implot/implot.cpp",
+];
+
+fn assert_file_exists(path: &str) -> io::Result<()> {
+    match fs::metadata(path) {
+        Ok(_) => Ok(()),
+        Err(ref e) if e.kind() == io::ErrorKind::NotFound => {
+            panic!(
+                "Can't access {}. Did you forget to fetch git submodules?",
+                path
+            );
+        }
+        Err(e) => Err(e),
+    }
+}
+
 fn main() -> io::Result<()> {
-    // Does nothing yet
+    let mut build = cc::Build::new();
+    build.cpp(true);
+    // Disabled due to linking issues
+    build
+        .define("CIMGUI_NO_EXPORT", None)
+        .define("IMGUI_DISABLE_WIN32_FUNCTIONS", None)
+        .define("IMGUI_DISABLE_OSX_FUNCTIONS", None);
+
+    // This won't seem to work yet
+    let imgui_third_party =
+        std::env::var_os("DEP_IMGUI_THIRD_PARTY").expect("No env var found for third_party folder");
+    println!("third party is {:?}", imgui_third_party);
+    build.include(imgui_third_party);
+
+    build.flag_if_supported("-Wno-return-type-c-linkage");
+    for path in &CPP_FILES {
+        assert_file_exists(path)?;
+        build.file(path);
+    }
+    build.compile("libcimplot.a");
     Ok(())
 }

implot-sys/third-party/cimplot

@@ -0,0 +1 @@
+Subproject commit 929f61d27b8b4e3c899b2a386679a7f4a82826ce