WIP Added default flags to fix alignment.

There's an issue with compiling C(++) code on 64-bit CPUs running 32-bit
OSes, which have 32-bit-aligned allocators. The resulting binary will
use aligned accesses in release mode.

See romanz/electrs#226 for an example of such
issue.

This change adds default flags setting alignment to whatever the pointer
alignment is. I realize the limitations of this change such as:

* Works in Cargo build scripts only (the major use case for `cc` crate)
* Is unnecessary if allocator actually supports 64 bit alingment
* Is incorrect if allocator supports smaller alignment than what's
  pointer width.

However, resolving this issue requires either matching on all possible
triples, which I don't have the time to implement or somehow fetching
the configuration for given triple, which I don't know how to do. This
is also an experiment to see if it even helps at all.

src/lib.rs

@@ -1620,6 +1620,24 @@ impl Build {
             }
         }
 
+        // This won't work outside of build scripts now, but should be helpful already
+        if let Some(target_ptr_width) = std::env::var("CARGO_CFG_TARGET_POINTER_WIDTH").map_err(|err| if let std::env::VarError::NotUnicode(_) = err { panic!("Invalid encoding of env var CARGO_CFG_TARGET_POINTER_WIDTH") }).ok() {
+            match cmd.family {
+                ToolFamily::Clang => {
+                    cmd.args.push(format!("-fnew-alignment={}", target_ptr_width).into());
+                    cmd.args.push(format!("-fmax-type-align={}", target_ptr_width).into());
+                },
+                ToolFamily::Gnu => {
+                    cmd.args.push(format!("-mstack-align={}", target_ptr_width).into());
+                    cmd.args.push(format!("-mdata-align={}", target_ptr_width).into());
+                    cmd.args.push(format!("-mconst-align={}", target_ptr_width).into());
+                },
+                ToolFamily::Msvc { .. } => {
+                    // I don't know what needs to be set up here.
+                },
+            }
+        }
+
         if target.contains("-ios") {
             // FIXME: potential bug. iOS is always compiled with Clang, but Gcc compiler may be
             // detected instead.