src: Disable use of getpass() when it is not available

For example, getpass is not available on Android platform.

Change-Id: I4c3039a3fcf188fd9343d625a551d268a4a20a93
diff --git a/wscript b/wscript
index 45dbef1..7107235 100644
--- a/wscript
+++ b/wscript
@@ -52,6 +52,7 @@
                    mandatory=False)
     conf.check_cxx(lib='rt', uselib_store='RT', define_name='HAVE_RT', mandatory=False)
     conf.check_cxx(cxxflags=['-fPIC'], uselib_store='PIC', mandatory=False)
+    conf.check_cxx(function_name='getpass', header_name='unistd.h', mandatory=False)
 
     conf.check_osx_security(mandatory=False)