libtool.at: Fix failures for 'test silencing warnings'

Replace 'g++' with variable '$CXX' and add version number for flag
'-version-info'.

tests/libtool.at

diff --git a/tests/libtool.at b/tests/libtool.at
index 7143a50..770a57a 100755
--- a/tests/libtool.at
+++ b/tests/libtool.at
@@ -251,9 +251,9 @@
 void f(int *p) { *p = 21; }
 ]])
 
-AT_CHECK([$LIBTOOL --mode=compile --tag=CXX g++ -c x.cpp], [0], [stdout], [stderr])
+AT_CHECK([$LIBTOOL --mode=compile --tag=CXX $CXX -c x.cpp], [0], [stdout], [stderr])
 
-AT_CHECK([$LIBTOOL --no-warnings --mode=link --tag=CXX g++ -o libx.la -no-canonical-prefixes -R /usr/lib64/ -version-info x.lo], [0], [stdout], [stderr])
+AT_CHECK([$LIBTOOL --no-warnings --mode=link --tag=CXX $CXX -o libx.la -no-canonical-prefixes -R /usr/lib64/ -version-info 0:0:0 x.lo], [0], [stdout], [stderr])
 
 AT_CHECK([$GREP -- 'warning' stderr], [1], [ignore])