libelf: Cygwin fix for UNC paths

If destdir was / and prefix began with /
then we would attempt to install libelf
to a path beginning with // which is a
UNC path on Cygwin. This is generally
incorrect.

Signed-off-by: Ray Donnelly <mingw.android@gmail.com>

scripts/build/companion_libs/200-libelf.sh

@@ -135,6 +135,13 @@ do_libelf_backend() {
     CT_DoExecLog ALL ${make}
 
     CT_DoLog EXTRA "Installing libelf"
+
+    # Guard against $destdir$prefix == //
+    # which is a UNC path on Cygwin/MSYS2
+    if [[ ${destdir} == / ]] && [[ ${prefix} == /* ]]; then
+        destdir=
+    fi
+
     CT_DoExecLog ALL ${make} instroot="${destdir}" install
 }