Fix determination of when to use precompiled headers.
https://sourceforge.net/p/rsyncrypto/bugs/19/
https://sourceforge.net/p/rsyncrypto/bugs/21/
--- configure.orig	2017-09-06 12:49:12.000000000 -0500
+++ configure	2020-07-07 17:30:45.000000000 -0500
@@ -5551,7 +5551,7 @@
 
 touch conftest.h
 rm -f a.out
-if $CXX -c conftest.h >/dev/null 2>&1 && test "! -e a.out -a -f conftest.h.gch"
+if test "x$enable_dependency_tracking" != xno && $CXX $CXXFLAGS -c conftest.h >/dev/null 2>&1 && test ! -e a.out -a -f conftest.h.gch
 then
      if true; then
   PRECOMPILED_HEADERS_TRUE=