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

t/111-compile2/CC.do

@@ -3,7 +3,7 @@ redo-ifchange config.sh
 exec >$3
 cat <<-EOF
 	redo-ifchange \$2.c
-	gcc $CFLAGS -MD -MF \$3.deps -o \$3 -c \$2.c
+	cc $CFLAGS -MD -MF \$3.deps -o \$3 -c \$2.c
 	read DEPS <\$3.deps
 	rm -f \$3.deps
 	redo-ifchange \${DEPS#*:}

t/111-compile2/hello.do

@@ -1,4 +1,4 @@
 DEPS="main.o
 mystr.o"
 redo-ifchange $DEPS
-gcc -o $3 $DEPS
+cc -o $3 $DEPS