Makefile Upgrade

[milfeld]

Changes work on TACC's Frontera machine and Mac (OS X).

• Uses VPATH for finding cpp file in different directories -- this simplifies rules
• Aborts if gsl-config isn't available
• Fixed (INTEL) compiler search 0=found | 1=notfound ; makes conditional simple (ifeq 0|1)
• Also uses conditional for GCC
• For objects, uses basename to get base file name
• Cleaned up directory prefix (shorten variable names and group)
• Simplified obj and bin rule logic and readability.
• put rules in canonical order
  -- now the makefile looks a bit cleaner