make compiler tunable in Makefile, i think potentially nice and useful

Makefile

@@ -1,13 +1,16 @@
+# choose your compiler, e.g. gcc/clang
+# example override to clang: make run CC=clang
+CC = gcc
 
 # the most basic way of building that is most likely to work on most systems
 .PHONY: run
 run: run.c
-	gcc -O3 -o run run.c -lm
+	$(CC) -O3 -o run run.c -lm
 
 # useful for a debug build, can then e.g. analyze with valgrind, example:
 # $ valgrind --leak-check=full ./run out/model.bin 1.0 3
 rundebug: run.c
-	gcc -g -o run run.c -lm
+	$(CC) -g -o run run.c -lm
 
 # https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html
 # https://simonbyrne.github.io/notes/fastmath/
@@ -20,14 +23,14 @@ rundebug: run.c
 # In our specific application this is *probably* okay to use
 .PHONY: runfast
 runfast: run.c
-	gcc -Ofast -o run run.c -lm
+	$(CC) -Ofast -o run run.c -lm
 
 # additionally compiles with OpenMP, allowing multithreaded runs
 # make sure to also enable multiple threads when running, e.g.:
 # OMP_NUM_THREADS=4 ./run out/model.bin
 .PHONY: runomp
 runomp: run.c
-	gcc -Ofast -fopenmp -march=native run.c  -lm  -o run
+	$(CC) -Ofast -fopenmp -march=native run.c  -lm  -o run
 
 .PHONY: clean
 clean: