Correctly handle external *FLAGS. Fixes #10.

Makefile

@@ -2,22 +2,30 @@ CC?=gcc
 CXX?=g++
 AR?=ar
 
+# Set the default optional flags if none are specified
+
 # Release mode (If just dropping the lib into your project, check out -flto too.)
 #
-# Note1: OpenMP is (currently) not required by the lib, just benchmarking.
+# Note1: OpenMP is (currently) not required by the lib, just for precise benchmarking.
 # Note2: the -Wa,-ahl=... part only generates .s assembly so one can see generated code.
-# Note3: When using `-flto`, you should add the same -O to LDFLAGS as to FLAGS.
-FLAGS=-fPIC -Wall -Wextra -I. -O3 -g -DNDEBUG -fopenmp -Wa,-ahl=$(@:.o=.s)
-LDFLAGS=-fopenmp -O3 -lm
+# Note3: If you want to add `-flto`, you should add the same -O to LDFLAGS as to FLAGS.
+DEFAULT_FLAGS=-O3 -g -DNDEBUG -fopenmp -Wall -Wextra -Wa,-ahl=$(@:.o=.s)
+DEFAULT_LDFLAGS=-fopenmp
 
 # Debug mode
-# FLAGS=-fPIC -Wall -Wextra -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC -I. -O0 -g -fopenmp -Wa,-ahl=$(@:.o=.s)
-# LDFLAGS=-fopenmp -O0 -g -lm
-# TODO: Play with -D_GLIBCXX_PARALLEL
-# TODO: Play with -D_GLIBCXX_PROFILE
+# DEFAULT_FLAGS=-fPIC -Wall -Wextra -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC -I. -O0 -g -fopenmp -Wa,-ahl=$(@:.o=.s)
+# DEFAULT_LDFLAGS=-fopenmp -g
+
+# First set the flags to their defaults if not supplied externally.
+CFLAGS?=$(DEFAULT_FLAGS)
+CXXFLAGS?=$(DEFAULT_FLAGS)
+LDFLAGS?=$(DEFAULT_LDFLAGS)
+
+# Then add those flags we can't live without, unconditionally.
+CFLAGS+=-fPIC -I. -pedantic
+CXXFLAGS+=-fPIC -I. -std=c++0x
+LDFLAGS+=-lm
 
-CFLAGS=$(FLAGS) -pedantic
-CXXFLAGS=$(FLAGS) -std=c++0x
 
 .PHONY: all benchmarks samples clean