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.

Tests/GeneratorExpression/CMakeLists.txt

@@ -0,0 +1,26 @@
+cmake_minimum_required (VERSION 2.8.8)
+project(GeneratorExpression NONE)
+
+add_custom_target(check ALL
+  COMMAND ${CMAKE_COMMAND}
+    -Dtest_0=$<0:nothing>
+    -Dtest_1=$<1:content>
+    -Dconfig=$<CONFIGURATION>
+    -Dtest_and_0=$<AND:0>
+    -Dtest_and_0_0=$<AND:0,0>
+    -Dtest_and_0_1=$<AND:0,1>
+    -Dtest_and_1=$<AND:1>
+    -Dtest_and_1_0=$<AND:1,0>
+    -Dtest_and_1_1=$<AND:1,1>
+    -Dtest_not_0=$<NOT:0>
+    -Dtest_not_1=$<NOT:1>
+    -Dtest_or_0=$<OR:0>
+    -Dtest_or_0_0=$<OR:0,0>
+    -Dtest_or_0_1=$<OR:0,1>
+    -Dtest_or_1=$<OR:1>
+    -Dtest_or_1_0=$<OR:1,0>
+    -Dtest_or_1_1=$<OR:1,1>
+    -P ${CMAKE_CURRENT_SOURCE_DIR}/check.cmake
+  COMMAND ${CMAKE_COMMAND} -E echo "check done"
+  VERBATIM
+  )

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")