Tests: use cc instead of gcc.

This fixes problems where llvm is installed without a 'gcc' alias.

t/110-compile/CC.do

@@ -1,5 +1,5 @@
 exec >$3
 cat <<-EOF
-	gcc -Wall -o /dev/fd/1 -c "\$1"
+	cc -Wall -o /dev/fd/1 -c "\$1"
 EOF
 chmod a+x $3

t/110-compile/LD.do

@@ -2,6 +2,6 @@ exec >$3
 cat <<-EOF
 	OUT="\$1"
 	shift
-	gcc -Wall -o "\$OUT" "\$@"
+	cc -Wall -o "\$OUT" "\$@"
 EOF
 chmod a+x $3

t/110-compile/yellow.o.do

@@ -1,3 +1,3 @@
 redo-ifchange CC hello.c
 ../sleep 2
-gcc -o $3 -c hello.c
+cc -o $3 -c hello.c