do not error out on generic-gnu + --enable-shared

If you build with --enabled-shared on a Linux arch not explicitly
listed, the configure script will abort because it didn't detect
"linux" in the fallback generic-gnu tuple.

Since this is the fallback tuple and people are passing
--enable-shared, assume the user knows what they're in for.

Change-Id: Ia35b657e7247c8855e3a94fca424c9884d4241e3

configure

diff --git a/configure b/configure
index 2e19e5b..dde215f 100755
--- a/configure
+++ b/configure
@@ -454,7 +454,13 @@
         # 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.
-        enabled linux || die "--enable-shared only supported on ELF for now"
+        if ! enabled linux; then
+            if enabled gnu; then
+                echo "--enable-shared is only supported on ELF; assuming this is OK"
+            else
+                die "--enable-shared only supported on ELF for now"
+            fi
+        fi
     fi
     if [ -z "$CC" ]; then
         echo "Bypassing toolchain for environment detection."