cmTarget: Add interface for compatible numeric properties

When using the boost MPL library, one can set a define to increase
the limit of how many variadic elements should be supported. The
default for BOOST_MPL_LIMIT_VECTOR_SIZE is 20:

 http://www.boost.org/doc/libs/1_36_0/libs/mpl/doc/refmanual/limit-vector-size.html

If the foo library requires that to be set to 30, and the independent
bar library requires it to be set to 40, consumers of both need to set
it to 40.

 add_library(foo INTERFACE)
 set_property(TARGET foo PROPERTY INTERFACE_boost_mpl_vector_size 30)
 set_property(TARGET foo PROPERTY COMPATIBLE_INTERFACE_NUMBER_MAX boost_mpl_vector_size)
 target_compile_definitions(foo INTERFACE BOOST_MPL_LIMIT_VECTOR_SIZE=$<TARGET_PROPERTY:boost_mpl_vector_size>)

 add_library(bar INTERFACE)
 set_property(TARGET bar PROPERTY INTERFACE_boost_mpl_vector_size 40)
 # Technically the next two lines are redundant, but as foo and bar are
 # independent, they both set these interfaces.
 set_property(TARGET bar PROPERTY COMPATIBLE_INTERFACE_NUMBER_MAX boost_mpl_vector_size)
 target_compile_definitions(bar INTERFACE BOOST_MPL_LIMIT_VECTOR_SIZE=$<TARGET_PROPERTY:boost_mpl_vector_size>)

 add_executable(user)
 target_link_libraries(user foo bar)

Because the TARGET_PROPERTY reads the boost_mpl_vector_size property
from the HEAD of the dependency graph (the user target), and because
that property appears in the COMPATIBLE_INTERFACE_NUMBER_MAX of
the dependencies of the user target, the maximum value for it is
chosen for the compile definition, ie, 40.

There are also use-cases for choosing the minimum value of a number.
In Qt, deprecated API can be disabled by version. Setting the
definition QT_DISABLE_DEPRECATED_BEFORE=0 disables no deprecated
API. Setting it to 0x501000 disables API which was deprecated before
Qt 5.1 etc.

If two dependencies require the use of API which was deprecated in
different Qt versions, then COMPATIBLE_INTERFACE_NUMBER_MIN can be
used to ensure that both can compile.

1 files changed, 3 insertions, 2 deletions

diff --git a/Help/prop_tgt/COMPATIBLE_INTERFACE_STRING.rst b/Help/prop_tgt/COMPATIBLE_INTERFACE_STRING.rst
index a461f76..f3fb43d 100644
--- a/Help/prop_tgt/COMPATIBLE_INTERFACE_STRING.rst
+++ b/Help/prop_tgt/COMPATIBLE_INTERFACE_STRING.rst
@@ -11,5 +11,6 @@ property "FOO" appears in the list, then for each dependee, the
 equal with each other, and with the "FOO" property in the dependee.
 If the property is not set, then it is ignored.  Note that for each
 dependee, the set of properties from this property must not intersect
-with the set of properties from the COMPATIBLE_INTERFACE_BOOL
-property.
+with the set of properties from the :prop_tgt:`COMPATIBLE_INTERFACE_BOOL`,
+:prop_tgt:`COMPATIBLE_INTERFACE_NUMBER_MIN` or
+:prop_tgt:`COMPATIBLE_INTERFACE_NUMBER_MAX` property.