bdw-gc: build static library

Many libraries (e.g. libyaml, libevent) available on Homebrew
contain both the static and dynamic versions of the library.
bdw-gc by default only compiles the dynamic version. This
commit enables building the static libgc.a to allow for
consumers to statically link this library in their applications.

One primary use-case is programs written in Crystal, which uses
bdw-gc for garbage collection by default.

Closes #38359.

Signed-off-by: FX Coudert <fxcoudert@gmail.com>

Formula/bdw-gc.rb

@@ -26,7 +26,8 @@ class BdwGc < Formula
     system "./configure", "--disable-debug",
                           "--disable-dependency-tracking",
                           "--prefix=#{prefix}",
-                          "--enable-cplusplus"
+                          "--enable-cplusplus",
+                          "--enable-static"
     system "make"
     system "make", "check"
     system "make", "install"