configure.ac: honour --disable-libunwind if the .pc file is present

We should check the presence in order to determine if we should
[implicitly] set the CFLAGS/LIBS

v2: Drop spurious OMX hunk (Eric)

Cc: Eric Anholt <eric@anholt.net>
Reported-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Emil Velikov <emil.velikov@collabora.com>
diff --git a/configure.ac b/configure.ac
index ef19733..ab33ca0 100644
--- a/configure.ac
+++ b/configure.ac
@@ -1074,12 +1074,13 @@
         [LIBUNWIND="$enableval"],
         [LIBUNWIND="auto"])
 
-PKG_CHECK_MODULES(LIBUNWIND, libunwind, [HAVE_LIBUNWIND=yes], [HAVE_LIBUNWIND=no])
+PKG_CHECK_EXISTS(libunwind, [HAVE_LIBUNWIND=yes], [HAVE_LIBUNWIND=no])
 if test "x$LIBUNWIND" = "xauto"; then
     LIBUNWIND="$HAVE_LIBUNWIND"
 fi
 
 if test "x$LIBUNWIND" = "xyes"; then
+    PKG_CHECK_MODULES(LIBUNWIND, libunwind)
     if test "x$HAVE_LIBUNWIND" != "xyes"; then
         AC_MSG_ERROR([libunwind requested but not installed.])
     fi