Allow setting `CC' in Unix build (#58051).

* builds/unix/unix-cc.in (CC): Use `override'.  The command line
  value of `CC' (if any) is stored already in `CCraw'.

ChangeLog

@@ -1,3 +1,10 @@
+2020-04-08  Werner Lemberg  <wl@gnu.org>
+
+	Allow setting `CC' in Unix build (#58051).
+
+	* builds/unix/unix-cc.in (CC): Use `override'.  The command line
+	  value of `CC' (if any) is stored already in `CCraw'.
+
 2020-04-04  Nikhil Ramakrishnan  <ramakrishnan.nikhil@gmail.com>
 
 	[woff2] Return if single stream operation fails.

builds/unix/unix-cc.in

@@ -87,8 +87,12 @@ ANSIFLAGS := @XX_ANSIFLAGS@
 
 # C compiler to use -- we use libtool!
 #
-CCraw := $(CC)
-CC    := $(LIBTOOL) --mode=compile $(CCraw)
+# CC might be set on the command line; we store this value in `CCraw'.
+# Consequently, we use the `override' directive to ensure that the
+# libtool call is always prepended.
+#
+CCraw       := $(CC)
+override CC := $(LIBTOOL) --mode=compile $(CCraw)
 
 # Resource compiler to use on Cygwin/MinGW, usually windres.
 #