Merge bitcoin#14036: travis: Run unit tests --with-sanitizers=undefined

fad8afa travis: Run unit tests --with-sanitizers=undefined (MarcoFalke)
0d00fd5 depends: allow CC/CXX to be overridden during configure (Cory Fields)

Pull request description:

  See issue bitcoin#12691

Tree-SHA512: 0772e8e037411cb4664270945a0fc9c0c1ed24eeaf5308460baee707868f7b700d8e740a6babb1d02bc818a8a4b71d9e33601ab2cc850ff315755fcf3d79d29e

# Conflicts:
#	.travis.yml

depends/config.site.in

@@ -55,9 +55,12 @@ export PKG_CONFIG_PATH=$depends_prefix/share/pkgconfig
 CPPFLAGS="-I$depends_prefix/include/ $CPPFLAGS"
 LDFLAGS="-L$depends_prefix/lib $LDFLAGS"
 
-CC="@CC@"
-CXX="@CXX@"
-OBJC="${CC}"
+if test -n "@CC@" -a -z "${CC}"; then
+  CC="@CC@"
+fi
+if test -n "@CXX@" -a -z "${CXX}"; then
+  CXX="@CXX@"
+fi
 PYTHONPATH=$depends_prefix/native/lib/python/dist-packages:$PYTHONPATH
 
 if test -n "@AR@"; then