Add policy CMP0019 to skip include/link variable re-expansion

Historically CMake has always expanded ${} variable references in the
values given to include_directories(), link_directories(), and
link_libraries().  This has been unnecessary since general ${}
evaluation syntax was added to the language a LONG time ago, but has
remained for compatibility with VERY early CMake versions.

For a long time the re-expansion was a lightweight operation because it
was only processed once at the directory level and the fast-path of
cmMakefile::ExpandVariablesInString was usually taken because values did
not have any '$' in them.  Then commit d899eb71 (Call
ExpandVariablesInString for each target's INCLUDE_DIRECTORIES,
2012-02-22) made the operation a bit heavier because the expansion is
now needed on a per-target basis.  In the future we will support
generator expressions in INCLUDE_DIRECTORIES with $<> syntax, so the
fast-path in cmMakefile::ExpandVariablesInString will no longer be taken
and re-expansion will be very expensive.

Add policy CMP0019 to skip the re-expansion altogether in NEW behavior.
In OLD behavior perform the expansion but improve the fast-path
heuristic to match ${} but not $<>.  If the policy is not set then warn
if expansion actually does anything.  We expect this to be encountered
very rarely in practice.

1 files changed, 2 insertions, 0 deletions

diff --git a/Tests/RunCMake/CMP0019/CMP0019-NEW.cmake b/Tests/RunCMake/CMP0019/CMP0019-NEW.cmake
new file mode 100644
index 0000000000..3091e66ac7
--- /dev/null
+++ b/Tests/RunCMake/CMP0019/CMP0019-NEW.cmake
@@ -0,0 +1,2 @@
+cmake_policy(SET CMP0019 NEW)
+include(CMP0019-code.cmake)