Add code to CI to verify signed/unsigned char

Make sure that our attempt to test both signed and unsigned char is
actually right.

build-scripts/build-linux

@@ -8,6 +8,8 @@ cmake -S . -B build -DCI_MODE=1 -DCMAKE_BUILD_TYPE=RelWithDebInfo \
       -DREQUIRE_CRYPTO_OPENSSL=1 -DREQUIRE_CRYPTO_GNUTLS=1
 cmake --build build -j$(nproc) -- -k
 cd build
+# Make sure char is signed by default -- see also test-unsigned-char
+./qpdf/test_char_sign | grep 'char is signed'
 # libtests automatically runs with all crypto providers.
 env QPDF_TEST_COMPARE_IMAGES=1 ctest --verbose
 # Run just qpdf tests with remaining crypto providers.

build-scripts/test-unsigned-char

@@ -15,4 +15,8 @@ env CFLAGS="-funsigned-char" \
     -DENABLE_QTC=1
 cmake --build build -j$(nproc) -- -k
 cd build
+# Make sure char is unsigned by default. ./build-linux verifies that
+# that build has char signed by default. That way we can be sure we
+# are testing both ways.
+./qpdf/test_char_sign | grep 'char is unsigned'
 ctest --verbose

qpdf/CMakeLists.txt

@@ -3,6 +3,7 @@ set(MAIN_CXX_PROGRAMS
   fix-qdf
   pdf_from_scratch
   sizes
+  test_char_sign
   test_driver
   test_large_file
   test_many_nulls

qpdf/test_char_sign.cc

@@ -0,0 +1,11 @@
+#include <cstdio>
+int main()
+{
+    char ch = '\xf7';
+    if (ch < 0) {
+        printf("char is signed\n");
+    } else {
+        printf("char is unsigned\n");
+    }
+    return 0;
+}