1. Ensure to override CFLAGS properly. Move the setting of CFLAGS outside the AM_CONDITIONAL so that Automake doesn't get confused (because CFLAGS is already set inside an AM_CONDITIONAL -- moving it outside the conditional ensure that this local CFLAGS override trumps all other CFLAGS overrides).
2. Only build libfabric on Linux. Add a little more configury to ensure that we only try to build libfabric on Linux.
3. Remove a dead/unused file
4. Fix typo in condition check
5. Use "false", not "/bin/false"
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