Add --enable-shared option to iosbuild.sh to build dynamic framework

Also allows use of --enable-shared when configuring for Mac OS X,
producing a bare .dylib.

Enabling the shared framework bumps the iOS deployment target to 8.0,
the minimum required to support dynamic framework deployment in apps.

When not using --enable-shared, a static library for iOS 6.0+ will still
be built.

Minimum version settings have been moved into ios-version.sh so they
can be updated in a single place.

As with the static build, unless header search paths are manually
tweaked, users must add a VPX prefix on includes, such as:

  #include <VPX/vpx/vpx_decoder.h>

A module map for headers is not yet included as inttypes.h is not
modular; this means that VPX cannot be used directly in Swift code,
but can still be pulled in through an Objective-C wrapper.

BUG=https://bugs.chromium.org/p/webm/issues/detail?id=1092
Change-Id: I28fb06ce65e48ed167a88c14a7bfb2861989317e
diff --git a/configure b/configure
index 91407d3..04ea0f4 100755
--- a/configure
+++ b/configure
@@ -515,13 +515,18 @@
         # Can only build shared libs on a subset of platforms. Doing this check
         # here rather than at option parse time because the target auto-detect
         # magic happens after the command line has been parsed.
-        if ! enabled linux && ! enabled os2; then
+        case "${tgt_os}" in
+        linux|os2|darwin*|iphonesimulator*)
+            # Supported platforms
+            ;;
+        *)
             if enabled gnu; then
                 echo "--enable-shared is only supported on ELF; assuming this is OK"
             else
-                die "--enable-shared only supported on ELF and OS/2 for now"
+                die "--enable-shared only supported on ELF, OS/2, and Darwin for now"
             fi
-        fi
+            ;;
+        esac
     fi
     if [ -z "$CC" ] || enabled external_build; then
         echo "Bypassing toolchain for environment detection."