Add option to use system cram for tests

If you already have cram installed, and you prefer to use the
system-installed copy instead of the copy we've embedded, you can now do
so by setting the `CRAM_EXECUTABLE` variable.  Its value should be the
full path to the system-installed cram executable, which is typically
/usr/bin/cram.

tests/CMakeLists.txt

@@ -98,6 +98,10 @@ make_u128_suite(sub)
 #-----------------------------------------------------------------------
 # Command-line tests
 
+if (NOT CRAM_EXECUTABLE)
+    set(CRAM_EXECUTABLE "${CMAKE_CURRENT_SOURCE_DIR}/cram.py")
+endif (NOT CRAM_EXECUTABLE)
+
 if (PYTHON_EXECUTABLE)
     configure_file(ccram ${CMAKE_BINARY_DIR}/ccram COPYONLY)
     file(GLOB_RECURSE TESTS "${CMAKE_CURRENT_SOURCE_DIR}/*.t")
@@ -109,6 +113,7 @@ if (PYTHON_EXECUTABLE)
             ${CMAKE_BINARY_DIR}/ccram
                 --python ${PYTHON_EXECUTABLE}
                 --root ${CMAKE_SOURCE_DIR}
+                --cram ${CRAM_EXECUTABLE}
                 --tests ${TEST}
         )
     endforeach(TEST)

tests/ccram

@@ -1,5 +1,7 @@
 #!/bin/sh
 
+set -e
+
 if [ "$1" = "--python" ]; then
     shift
     PYTHON="$1"
@@ -16,6 +18,14 @@ else
     ROOT=$(dirname $PWD)
 fi
 
+if [ "$1" = "--cram" ]; then
+    shift
+    CRAM="$1"
+    shift
+else
+    CRAM="$ROOT/tests/cram.py"
+fi
+
 if [ "$1" = "--tests" ]; then
     shift
     TESTS="$1"
@@ -27,5 +37,4 @@ fi
 export ROOT
 
 LD_LIBRARY_PATH="$PWD/src:$LD_LIBRARY_PATH" \
-PATH="$PWD/src:$PATH" \
-  "$PYTHON" "$ROOT/tests/cram.py" "$@" "$TESTS"
+PATH="$PWD/src:$PATH" exec "$PYTHON" "$CRAM" "$@" "$TESTS"