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.
```
This was detected while running the tests with the `-Wconditional-uninitialized` flag
```
./autogen.sh
CC=clang CFLAGS="-Wconditional-uninitialized" ./configure
make check
```
The resulting warning is a false positive, but setting the value to -1
ensures that the CHECK below will fail if recid is never written to.
PiRK