AC_CHECK_HEADERS should come before AC_SEARCH_LIBS

the latter can make use of the result of the former

Also, annoyingly, AC_CHECK_HEADERS will call the action-if-not-found for
*every* header it its list that is not found (as opposed to just if no
header in the list is found.  So we should set sage_spkg_install_libffi=no again in case the second header is found but not the first.

build/pkgs/libffi/spkg-configure.m4

@@ -2,8 +2,8 @@ SAGE_SPKG_CONFIGURE([libffi], [
     dnl First try checking for libffi with pkg-config
     PKG_CHECK_MODULES([LIBFFI], [libffi], [], [
         dnl Fallback to manually grubbing around for headers and libs
+        AC_CHECK_HEADERS([ffi.h ffi/ffi.h], [sage_spkg_install_libffi=no; break], [sage_spkg_install_libffi=yes])
         AC_SEARCH_LIBS([ffi_call], [ffi], [], [sage_spkg_install_libffi=yes])
-        AC_CHECK_HEADERS([ffi.h ffi/ffi.h], [break], [sage_spkg_install_libffi=yes])
     ])
 ])