cmake: Teach --build to reject multiple --target options

Previously we did not clearly document that `--target` is only supported
to be specified once.  Even worse, specifying it multiple times would
silently ignore any previously specified targets and only build the last
target.

Update the documentation to specify this.  Update the implementation to
reject multiple `--target` options to prevent user errors.

2 files changed, 7 insertions, 0 deletions

diff --git a/Help/manual/cmake.1.rst b/Help/manual/cmake.1.rst
index 273a780..959148e 100644
--- a/Help/manual/cmake.1.rst
+++ b/Help/manual/cmake.1.rst
@@ -58,6 +58,7 @@ Options
 
    <dir>          = Project binary directory to be built.
    --target <tgt> = Build <tgt> instead of default targets.
+                    May only be specified once.
    --config <cfg> = For multi-configuration tools, choose <cfg>.
    --clean-first  = Build target 'clean' first, then build.
                     (To clean only, use --target 'clean'.)
diff --git a/Help/release/dev/error-multiple-targets.rst b/Help/release/dev/error-multiple-targets.rst
new file mode 100644
index 0000000..060b26b
--- /dev/null
+++ b/Help/release/dev/error-multiple-targets.rst
@@ -0,0 +1,6 @@
+error-multiple-targets
+----------------------
+
+* The :manual:`cmake(1)` ``--build`` command-line tool now rejects multiple
+  ``--target`` options with an error instead of silently ignoring all but the
+  last one.