Allow the library consumer to specify the Android API level

The build.rs script will use the C preprocessor to pull the API level
out of C headers by default. But for build systems that wish to control
each build step, and rely on remote compilation, this presents problems.

Remove the requirement to run the C preprocessor if the API level is
specified via `--cfg`:
* android_api_at_least_0: No minimum API level is guaranteed.
* android_api_at_least_21: The API level will be at least 21.

build.rs

@@ -1,5 +1,3 @@
-extern crate cc;
-
 use std::env;
 use std::path::Path;
 
@@ -11,14 +9,17 @@ pub fn main() {
     }
 }
 
-// Used to detect the value of the `__ANDROID_API__`
-// builtin #define
-const MARKER: &str = "BACKTRACE_RS_ANDROID_APIVERSION";
-const ANDROID_API_C: &str = "
+#[cfg(not(any(android_api_at_least_0, android_api_at_least_21)))]
+fn android_version_from_c_headers() -> Option<u32> {
+    extern crate cc;
+
+    // Used to detect the value of the `__ANDROID_API__`
+    // builtin #define
+    const MARKER: &str = "BACKTRACE_RS_ANDROID_APIVERSION";
+    const ANDROID_API_C: &str = "
 BACKTRACE_RS_ANDROID_APIVERSION __ANDROID_API__
 ";
 
-fn build_android() {
     // Create `android-api.c` on demand.
     // Required to support calling this from the `std` build script.
     let out_dir = env::var_os("OUT_DIR").unwrap();
@@ -32,27 +33,55 @@ fn build_android() {
                 "warning: android version detection failed while running C compiler: {}",
                 e
             );
-            return;
+            return None;
         }
     };
     let expansion = match std::str::from_utf8(&expansion) {
         Ok(s) => s,
-        Err(_) => return,
+        Err(_) => return None,
     };
     eprintln!("expanded android version detection:\n{}", expansion);
     let i = match expansion.find(MARKER) {
         Some(i) => i,
-        None => return,
+        None => return None,
     };
     let version = match expansion[i + MARKER.len() + 1..].split_whitespace().next() {
         Some(s) => s,
-        None => return,
+        None => return None,
     };
-    let version = match version.parse::<u32>() {
-        Ok(n) => n,
-        Err(_) => return,
+    match version.parse::<u32>() {
+        Ok(n) => Some(n),
+        Err(_) => None,
+    }
+}
+
+/// Sets cfgs that depend on the Android API level.
+///
+/// This depends on the use of a C preprocessor to find the API level in system
+/// headers. For build systems that do not want to use a C processor inside the
+/// execution of build scripts, the build system can specify the API level
+/// through a `--cfg` flag. If any is set, the C preprocessor will not be used:
+/// * android_api_at_least_0: No minimum API level is guaranteed.
+/// * android_api_at_least_21: The API level will be at least 21.
+fn build_android() {
+    let at_least_21;
+    #[cfg(android_api_at_least_0)]
+    {
+        at_least_21 = false;
+    }
+    #[cfg(android_api_at_least_21)]
+    {
+        at_least_21 = true;
+    }
+    #[cfg(not(any(android_api_at_least_0, android_api_at_least_21)))]
+    {
+        at_least_21 = {
+            let version = android_version_from_c_headers().unwrap_or_default();
+            version >= 21
+        }
     };
-    if version >= 21 {
+
+    if at_least_21 {
         println!("cargo:rustc-cfg=feature=\"dl_iterate_phdr\"");
     }
 }