compiler wrappers: don't override -isystem with -I (#16077)

If the Spack compiler wrapper encounters any "-isystem" option, then
when adding include directories for Spack dependencies, Spack will
use "-isystem" instead of "-I". This prevents Spack-generated "-I"
options from overriding the "-isystem" options generated by the build
system. To ensure that build-system "-isystem" directories are
searched first, Spack places all of its inserted "-isystem"
directories after.

The new ordering of -isystem includes is:

* -isystem from build system (not system directories)
* -isystem from Spack
* -isystem from build system (for directories like /usr/include)

The prior order of "-I" arguments is preserved (although as of this
commit Spack no longer generates -I if -isystem is detected):

* -I from build system (not system directories)
* -I from Spack (only if there are no "-isystem" options)
* -I from build system (for directories like /usr/include)

0 files changed, 0 insertions, 0 deletions