Ninja: Centralize path conversion in global generator (#15757)

In the Ninja generator we run all build rules from the top of the build
tree rather than changing into each subdirectory.  Therefore we convert
all paths relative to the HOME_OUTPUT directory.  However, the Convert
method on cmLocalGenerator restricts relative path conversions to avoid
leaving the build tree with a "../" sequence.  Therefore conversions
performed for "subdirectories" that are outside the top of the build
tree always use full paths while conversions performed for
subdirectories that are inside the top of the build tree may use
relative paths to refer to the same files.

Since Ninja always runs rules from the top of the build tree we should
convert them using only the top-level cmLocalGenerator in order to
remain consistent.  Also extend the test suite with a case that fails
without this fix.

2 files changed, 3 insertions, 0 deletions

diff --git a/Tests/OutOfSource/SubDir/CMakeLists.txt b/Tests/OutOfSource/SubDir/CMakeLists.txt
index c5df36e..e18dbb9 100644
--- a/Tests/OutOfSource/SubDir/CMakeLists.txt
+++ b/Tests/OutOfSource/SubDir/CMakeLists.txt
@@ -6,3 +6,5 @@ add_subdirectory(${OutOfSource_SOURCE_DIR}/../OutOfBinary
 
 # subdir to a sibling dir
 add_subdirectory(${OutOfSource_SOURCE_DIR}/${KEN}OutOfSourceSubdir OutOfSourceSubdir )
+
+add_library(subdir subdir.c)
diff --git a/Tests/OutOfSource/SubDir/subdir.c b/Tests/OutOfSource/SubDir/subdir.c
new file mode 100644
index 0000000..0d0d827
--- /dev/null
+++ b/Tests/OutOfSource/SubDir/subdir.c
@@ -0,0 +1 @@
+int subdir(void) { return 0; }