build: avoid non-portable shell in test setup

POSIX states that 'a=1; a=2 b=$a command' has unspecified results
for the value of $b visible within command.  In particular, on
BSD, this resulted in PATH not picking up the in-test ssh.

* tests/Makefile.am (lv_abs_top_builddir): New macro.
(path_add, TESTS_ENVIRONMENT): Use it to avoid referring to an
environment variable set previously within the same command line.
Reported by Matthias Bolte.

tests/Makefile.am

@@ -259,13 +259,17 @@ TESTS += interfacexml2xmltest
 
 TESTS += cputest
 
-path_add = $$abs_top_builddir/daemon$(PATH_SEPARATOR)$$abs_top_builddir/tools$(PATH_SEPARATOR)$$abs_top_builddir/tests
-
 # NB, automake < 1.10 does not provide the real
 # abs_top_{src/build}dir or builddir variables, so don't rely
 # on them here. Fake them with 'pwd'
+# Also, BSD sh doesn't like 'a=b b=$$a', so we can't use an
+# intermediate shell variable, but must do all the expansion in make
+
+lv_abs_top_builddir=`cd '$(top_builddir)'; pwd`
+path_add = $(lv_abs_top_builddir)/daemon$(PATH_SEPARATOR)$(lv_abs_top_builddir)/tools$(PATH_SEPARATOR)$(lv_abs_top_builddir)/tests
+
 TESTS_ENVIRONMENT =				\
-  abs_top_builddir=`cd '$(top_builddir)'; pwd`	\
+  abs_top_builddir=$(lv_abs_top_builddir)	\
   abs_top_srcdir=`cd '$(top_srcdir)'; pwd`	\
   abs_builddir=`pwd`				\
   abs_srcdir=`cd '$(srcdir)'; pwd`		\