ci: Use modern CMake command options in ci_verify_cmake.sh

Due to raising the minimum CMake version to 3.14, we can now run the
CMake build verification (ci_verify_cmake.sh) from the libpng source
dir, just as in the other scripts (ci_verify_*.sh). This is possible
thanks to the modern CMake command options `cmake -B` and `cmake -S`.

We can finally simplify the implementation of ci_verify_cmake.sh by
removing a code complication that was annoying, and yet, necessary in
peculiar Bash-on-Windows setups.

Fun fact:
CMake version 3.14 was released on 2019-03-14. Reportedly, on purpose!

ci/ci_verify_cmake.sh

@@ -17,12 +17,6 @@ CI_OUT_DIR="$CI_TOPLEVEL_DIR/out"
 CI_BUILD_DIR="$CI_OUT_DIR/ci_verify_cmake.$CI_TARGET_SYSTEM.$CI_TARGET_ARCH.build"
 CI_INSTALL_DIR="$CI_OUT_DIR/ci_verify_cmake.$CI_TARGET_SYSTEM.$CI_TARGET_ARCH.install"
 
-# Keep the following relative paths in sync with the absolute paths.
-# We use them for the benefit of native Windows tools that might be
-# otherwise confused by the path encoding used by Bash-on-Windows.
-CI_BUILD_TO_SRC_RELDIR="../.."
-CI_BUILD_TO_INSTALL_RELDIR="../ci_verify_cmake.$CI_TARGET_SYSTEM.$CI_TARGET_ARCH.install"
-
 function ci_init_build {
     # Ensure that the mandatory variables are initialized.
     CI_CMAKE="${CI_CMAKE:-cmake}"
@@ -148,40 +142,33 @@ function ci_build {
     all_cmake_build_flags+=($CI_CMAKE_BUILD_FLAGS)
     all_ctest_flags+=($CI_CTEST_FLAGS)
     # And... build!
-    # Use $CI_BUILD_TO_SRC_RELDIR and $CI_BUILD_TO_INSTALL_RELDIR
-    # instead of $CI_SRC_DIR and $CI_INSTALL_DIR from this point onwards.
     ci_spawn mkdir -p "$CI_BUILD_DIR"
-    ci_spawn cd "$CI_BUILD_DIR"
-    [[ $CI_BUILD_TO_SRC_RELDIR -ef $CI_SRC_DIR ]] || {
-        ci_err_internal "bad or missing \$CI_BUILD_TO_SRC_RELDIR"
-    }
-    ci_spawn mkdir -p "$CI_INSTALL_DIR"
-    [[ $CI_BUILD_TO_INSTALL_RELDIR -ef $CI_INSTALL_DIR ]] || {
-        ci_err_internal "bad or missing \$CI_BUILD_TO_INSTALL_RELDIR"
-    }
     # Spawn "cmake ...".
-    ci_spawn "$CI_CMAKE" -DCMAKE_INSTALL_PREFIX="$CI_BUILD_TO_INSTALL_RELDIR" \
-                         "${all_cmake_vars[@]}" \
-                         "$CI_BUILD_TO_SRC_RELDIR"
+    ci_spawn "$CI_CMAKE" -B "$CI_BUILD_DIR" \
+                         -S . \
+                         -DCMAKE_INSTALL_PREFIX="$CI_INSTALL_DIR" \
+                         "${all_cmake_vars[@]}"
     # Spawn "cmake --build ...".
-    ci_spawn "$CI_CMAKE" --build . \
+    ci_spawn "$CI_CMAKE" --build "$CI_BUILD_DIR" \
                          --config "$CI_CMAKE_BUILD_TYPE" \
                          "${all_cmake_build_flags[@]}"
     ci_expr $((CI_NO_TEST)) || {
         # Spawn "ctest" if testing is not disabled.
+        ci_spawn pushd "$CI_BUILD_DIR"
         ci_spawn "$CI_CTEST" --build-config "$CI_CMAKE_BUILD_TYPE" \
                              "${all_ctest_flags[@]}"
+        ci_spawn popd
     }
     ci_expr $((CI_NO_INSTALL)) || {
         # Spawn "cmake --build ... --target install" if installation is not disabled.
-        ci_spawn "$CI_CMAKE" --build . \
+        ci_spawn "$CI_CMAKE" --build "$CI_BUILD_DIR" \
                              --config "$CI_CMAKE_BUILD_TYPE" \
                              --target install \
                              "${all_cmake_build_flags[@]}"
     }
     ci_expr $((CI_NO_CLEAN)) || {
         # Spawn "make --build ... --target clean" if cleaning is not disabled.
-        ci_spawn "$CI_CMAKE" --build . \
+        ci_spawn "$CI_CMAKE" --build "$CI_BUILD_DIR" \
                              --config "$CI_CMAKE_BUILD_TYPE" \
                              --target clean \
                              "${all_cmake_build_flags[@]}"