Respect CC/CXX, don't use GCC-only flags, don't append -g, don't
append -L/usr/local.

diff --git a/Makefile b/Makefile
index 1d5fad9..f3d7938 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,7 @@ ALL: default
 
 # When -g is used, it seems that stackdump does not work.
 # GPROFFLAG = -pg
-GPROFFLAG = -g
+#GPROFFLAG = -g
 #GPROFFLAG = 
 
 PREFIX?=/usr/local
@@ -63,7 +63,7 @@ CDDDEFINE_PREFIX = -DNOCDDPREFIX
 endif
 
 ifeq ($(cddpath),)
-CDD_LINKOPTIONS = -L/usr/local -lcddgmp
+CDD_LINKOPTIONS = -lcddgmp
 CDD_INCLUDEOPTIONS =
 else
 CDD_LINKOPTIONS = $(cddpath)/lib/libcddgmp.a
@@ -142,9 +142,9 @@ PREFIX =
 SHELL       = /bin/sh
 #ARCH        = LINUX
 
-CC          = $(PREFIX)gcc
+CC          ?= $(PREFIX)gcc
 CLINKER     = $(CC)
-CXX         = $(PREFIX)g++
+CXX         ?= $(PREFIX)g++
 CCLINKER    = $(CXX)
 
 #CC          = $(PREFIX)gcc-8.1
@@ -162,7 +162,7 @@ SYMMETRICTRAVERSAL_FLAGS = -fno-guess-branch-probability
 # -rdynamic is Linux-only; use TBB for parallel execution support
 PLATFORM_LINKOPTIONS = -L$(HOMEBREW_PREFIX)/opt/tbb/lib -ltbb
 else
-SYMMETRICTRAVERSAL_FLAGS = -fno-guess-branch-probability
+#SYMMETRICTRAVERSAL_FLAGS = -fno-guess-branch-probability
 PLATFORM_LINKOPTIONS = -rdynamic -ltbb
 endif