Add comments to Makefile.config for standalone

RecoTracker/MkFitCore/standalone/Makefile.config

@@ -128,8 +128,11 @@ else
 endif
 
 ifeq ($(CXX), g++)
+  # For gcc, compile with flags approximating "scram build echo_CXXFLAGS" in a CMSSW env
   CXXFLAGS += -std=c++1z -ftree-vectorize -Werror=main -Werror=pointer-arith -Werror=overlength-strings -Wno-vla -Werror=overflow -Wstrict-overflow -Werror=array-bounds -Werror=format-contains-nul -Werror=type-limits -fvisibility-inlines-hidden -fno-math-errno --param vect-max-version-for-alias-checks=50 -Xassembler --compress-debug-sections -felide-constructors -fmessage-length=0 -Wall -Wno-non-template-friend -Wno-long-long -Wreturn-type -Wunused -Wparentheses -Wno-deprecated -Werror=return-type -Werror=missing-braces -Werror=unused-value -Werror=address -Werror=format -Werror=sign-compare -Werror=write-strings -Werror=delete-non-virtual-dtor -Wstrict-aliasing -Werror=narrowing -Werror=unused-but-set-variable -Werror=reorder -Werror=unused-variable -Werror=conversion-null -Werror=return-local-addr -Wnon-virtual-dtor -Werror=switch -fdiagnostics-show-option -Wno-unused-local-typedefs -Wno-attributes -Wno-psabi
   CXXFLAGS += -fdiagnostics-color=auto -fdiagnostics-show-option -pthread -pipe -fopenmp-simd
+  # Add flags to enable auto-vectorization of sin/cos with libmvec (glibc >= 2.22)
+  # Disable reciprocal math for Intel/AMD consistency, see cms-sw/cmsdist#8280
   CXXFLAGS += -Ofast -fno-reciprocal-math -mrecip=none
 endif