botan: fix xcode-only install

- botan's configure.py ignores our CFLAGS and switches the compiler to
  llvm :-/ We can use the MACH_OPT variable to insert our CLFAGS and
  make this build with Xcode-only setups.
- Using ENV.compiler.to_s for the compiler string.
- Fixing the "ld: unknown option soname" issue when using clang.

Closes Homebrew/homebrew#13065.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Formula/botan.rb

@@ -13,8 +13,14 @@ class Botan < Formula
     args = ["--prefix=#{prefix}"]
     args << "--cpu=x86_64" if MacOS.prefer_64_bit?
     args << "--enable-debug" if ARGV.include? "--enable-debug"
+    # The --cc option needs "clang" or "gcc" and not the full path.
+    args << "--cc=#{ENV.compiler.to_s}"
 
     system "./configure.py", *args
-    system "make install"
+    # "-soname" can lead to issues like https://github.com/mxcl/homebrew/issues/11972
+    inreplace "Makefile", "-Wl,-soname", "-dynamiclib -install_name "
+    # A hack to force them use our CFLAGS. MACH_OPT is empty in the Makefile
+    # but used for each call to cc/ld.
+    system "make", "install", "MACH_OPT=#{ENV.cflags}"
   end
 end