Make name of QBE binary configurable via macro as well

Eases patching the code to refer to QBE by full path on Guix.

diff --git a/README b/README
index 67840799..1475edd7 100644
--- a/README
+++ b/README
@@ -208,6 +208,8 @@ file `include/bits/scc/sys.h` and it included basically 5 elements:
 
 	- ASBIN: macro with the name of the assembler binary.
 
+	- QBEBIN: macro with the name of the qbe binary.
+
 	- sysincludes: It is a list of diretories used to locate
 	  the system headers
 
diff --git a/include/scc/bits/scc/sys-musl.h b/include/scc/bits/scc/sys-musl.h
index 1cb1a12d..31267125 100644
--- a/include/scc/bits/scc/sys-musl.h
+++ b/include/scc/bits/scc/sys-musl.h
@@ -1,5 +1,6 @@
 #define LDBIN "ld"
 #define ASBIN "as"
+#define QBEBIN "qbe"
 
 /* configure below your system linker command line */
 #define GCCLIBPATH "/usr/lib/gcc/x86_64-unknown-linux-gnu/10.2/"
diff --git a/include/scc/bits/scc/sys-scc.h b/include/scc/bits/scc/sys-scc.h
index 9d6ae86b..2948c5b2 100644
--- a/include/scc/bits/scc/sys-scc.h
+++ b/include/scc/bits/scc/sys-scc.h
@@ -1,5 +1,6 @@
 #define LDBIN "ld"
 #define ASBIN "as"
+#define QBEBIN "qbe"
 
 /* configure below your standard sys include paths */
 char *sysincludes[] = {
diff --git a/src/cmd/scc-cc/posix/cc.c b/src/cmd/scc-cc/posix/cc.c
index 40a3c109..49372777 100644
--- a/src/cmd/scc-cc/posix/cc.c
+++ b/src/cmd/scc-cc/posix/cc.c
@@ -53,7 +53,7 @@ static struct tool {
 	[TEEIR]  = {.bin = "tee"},
 	[CC2]    = {.bin = "cc2"},
 	[TEEQBE] = {.bin = "tee"},
-	[QBE]    = {.bin = "qbe"},
+	[QBE]    = {.bin = QBEBIN},
 	[TEEAS]  = {.bin = "tee"},
 	[AS]     = {.bin = ASBIN},
 	[LD]     = {.bin = LDBIN},
@@ -239,7 +239,7 @@ settool(int tool, char *infile, int nexttool)
 			die("scc-cc: target tool path is too long");
 		break;
 	case QBE:
-		strcpy(t->cmd, "qbe");
+		strcpy(t->cmd, QBEBIN);
 
 		if (!strcmp(arch, "amd64") && !strcmp(abi, "sysv"))
 			fmt = "amd64_sysv";