Modified the definition of realpath() so that it does not generate
a python traceback when the argument is null.  Implementation by
Kareem Farid from pull request #391.

scripts/configure

diff --git a/scripts/configure b/scripts/configure
index ce26a4d..94ba333 100755
--- a/scripts/configure
+++ b/scripts/configure
@@ -2284,7 +2284,8 @@
 
 
 realpath() {
-    # Cross-platform, handles ~
+    # Cross-platform, handles empty argument, and ~
+    if [ "$#" != "1" ] ; then return ; fi
     python3 -c 'import os,sys;print(os.path.realpath(os.path.expanduser(sys.argv[1])))' $1
 }
 

scripts/configure.ac

diff --git a/scripts/configure.ac b/scripts/configure.ac
index 4b94fdd..9709351 100755
--- a/scripts/configure.ac
+++ b/scripts/configure.ac
@@ -16,7 +16,8 @@
 AC_SUBST(SED)
 
 realpath() {
-    # Cross-platform, handles ~
+    # Cross-platform, handles empty argument, and ~
+    if @<:@ "$#" != "1" @:>@ ; then return ; fi
     python3 -c 'import os,sys;print(os.path.realpath(os.path.expanduser(sys.argv@<:@1@:>@)))' $1
 }