Allow Makefile compiler to be set with CC variable

[ryandesign]

Instead of hardcoding "gcc", this allows the user to set the compiler with the typical CC environment variable, such as:

make CC=/path/to/my/clang

I initially also added a line CC=gcc at the top so as to not change the previous behavior. However make already provides a default value of "cc" for CC which is usually fine, since cc should be whatever the customary compiler is on a system. On macOS, since over a decade now, /usr/bin/gcc has not been gcc but rather a wrapper that calls clang (just as /usr/bin/cc does) and with which c2t appears to build fine. In fact, when building your macos targets, your Makefile doesn't even work with real gcc:

make macos CC=gcc-mp-11
...
gcc-mp-11 -Wall -Wno-strict-aliasing -Wno-unused-value -Wno-unused-function -I. -O3 -target x86_64-apple-macos10.12 -o bin/c2t_x86 c2t.c -lm
gcc-mp-11: error: unrecognized command-line option '-target'
make: *** [bin/c2t_x86] Error 1

For these reasons I felt that using make's default value of CC=cc was the way to go.