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/libs.mk b/libs.mk
index 54592ab..f563bd3 100644
--- a/libs.mk
+++ b/libs.mk
@@ -273,6 +273,12 @@
 LIBVPX_SO_SYMLINKS      := $(addprefix $(LIBSUBDIR)/, \
                              libvpx.dylib  )
 else
+ifeq ($(filter iphonesimulator%,$(TGT_OS)),$(TGT_OS))
+LIBVPX_SO               := libvpx.$(SO_VERSION_MAJOR).dylib
+SHARED_LIB_SUF          := .dylib
+EXPORT_FILE             := libvpx.syms
+LIBVPX_SO_SYMLINKS      := $(addprefix $(LIBSUBDIR)/, libvpx.dylib)
+else
 ifeq ($(filter os2%,$(TGT_OS)),$(TGT_OS))
 LIBVPX_SO               := libvpx$(SO_VERSION_MAJOR).dll
 SHARED_LIB_SUF          := _dll.a
@@ -288,6 +294,7 @@
                              libvpx.so.$(SO_VERSION_MAJOR).$(SO_VERSION_MINOR))
 endif
 endif
+endif
 
 LIBS-$(CONFIG_SHARED) += $(BUILD_PFX)$(LIBVPX_SO)\
                            $(notdir $(LIBVPX_SO_SYMLINKS)) \