print warnings for conditional-uninitialized

This compiler flag is available for clang but not gcc.

Test plan:

```
autogen.sh
./configure
make check
CC=clang ./configure
make check
```

If a variable is used uninitialized, the warning should look something
like:
```
  CC       src/tests-tests.o
src/tests.c:4336:15: warning: variable 'recid' may be uninitialized when used here [-Wconditional-uninitialized]
        CHECK(recid >= 0 && recid < 4);
              ^~~~~
./src/util.h:54:18: note: expanded from macro 'CHECK'
    if (EXPECT(!(cond), 0)) { \
                 ^~~~
./src/util.h:41:39: note: expanded from macro 'EXPECT'
                                      ^
src/tests.c:4327:14: note: initialize the variable 'recid' to silence this warning
    int recid;
             ^
              = 0
1 warning generated.
```

configure.ac

@@ -79,6 +79,15 @@ AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char foo;]])],
       CFLAGS="$saved_CFLAGS"
     ])
 
+saved_CFLAGS="$CFLAGS"
+CFLAGS="-Wconditional-uninitialized $CFLAGS"
+AC_MSG_CHECKING([if ${CC} supports -Wconditional-uninitialized])
+AC_COMPILE_IFELSE([AC_LANG_SOURCE([[char foo;]])],
+    [ AC_MSG_RESULT([yes]) ],
+    [ AC_MSG_RESULT([no])
+      CFLAGS="$saved_CFLAGS"
+    ])
+
 saved_CFLAGS="$CFLAGS"
 CFLAGS="-fvisibility=hidden $CFLAGS"
 AC_MSG_CHECKING([if ${CC} supports -fvisibility=hidden])