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, 23 insertions, 0 deletions

diff --git a/Tests/GeneratorExpression/check.cmake b/Tests/GeneratorExpression/check.cmake
new file mode 100644
index 0000000..c40b847
--- /dev/null
+++ b/Tests/GeneratorExpression/check.cmake
@@ -0,0 +1,23 @@
+macro(check var val)
+  if(NOT "${${var}}" STREQUAL "${val}")
+    message(SEND_ERROR "${var} is \"${${var}}\", not \"${val}\"")
+  endif()
+endmacro()
+
+message(STATUS "config=[${config}]")
+check(test_0 "")
+check(test_1 "content")
+check(test_and_0 "0")
+check(test_and_0_0 "0")
+check(test_and_0_1 "0")
+check(test_and_1 "1")
+check(test_and_1_0 "0")
+check(test_and_1_1 "1")
+check(test_not_0 "1")
+check(test_not_1 "0")
+check(test_or_0 "0")
+check(test_or_0_0 "0")
+check(test_or_0_1 "1")
+check(test_or_1 "1")
+check(test_or_1_0 "1")
+check(test_or_1_1 "1")