MNT: Add Makefile flag for conditional compilation against old-or-new FSL

config/default.mk

+# Boolean flag which defines this FSL installation
+# as being from FSL 6.0.6 or newer (a conda-based
+# FSL installation). This flag may be used by FSL
+# Makefiles which need to preserve support for
+# older versions of FSL.
+FSL_POST_606 := true
+
 # Platform specific settings and defaults,
 # which are used by vars.mk
 include ${FSLCONFDIR}/buildSettings.mk