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)
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