Add boolean generator expressions

Add generator expressions that combine and use boolean test results:

 $<0:...>         = empty string (ignores "...")
 $<1:...>         = content of "..."
 $<AND:?[,?]...>  = '1' if all '?' are '1', else '0'
 $<OR:?[,?]...>   = '0' if all '?' are '0', else '1'
 $<NOT:?>         = '0' if '?' is '1', else '1'

These will be useful to evaluate (future) boolean query expressions and
condition content on the results.  Include tests and documentation.

1 files changed, 17 insertions, 0 deletions

diff --git a/Tests/RunCMake/GeneratorExpression/BadAND-stderr.txt b/Tests/RunCMake/GeneratorExpression/BadAND-stderr.txt
new file mode 100644
index 0000000..ced21d8
--- /dev/null
+++ b/Tests/RunCMake/GeneratorExpression/BadAND-stderr.txt
@@ -0,0 +1,17 @@
+CMake Error at BadAND.cmake:1 \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<AND:>
+
+  AND requires one or more comma-separated '0' or '1' values.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)
++
+CMake Error at BadAND.cmake:1 \(add_custom_target\):
+  Error evaluating generator expression:
+
+    \$<AND:,>
+
+  AND requires one or more comma-separated '0' or '1' values.
+Call Stack \(most recent call first\):
+  CMakeLists.txt:3 \(include\)$