Document using GP_CAMLIB_SET on make command line

Documents the use of GP_CAMLIB_SET on the make command line.

Setting CAMLIBS= on the make command line to specify the set of
camlibs to be built and installed has not worked at least since
the introduction of GP_CAMLIB_SET.

HACKING.md

@@ -127,9 +127,12 @@ camlibs, you can use
 
 and
 
-    make CAMLIBS="sillycam.la doofuscam.la" install-camlibs
+    make GP_CAMLIB_SET="sillycam.la doofuscam.la" install-camlibs
 
-respectively.
+respectively or
+
+    make -C camlibs GP_CAMLIB_SET="sillycam.la doofuscam.la" all
+    make -C camlibs GP_CAMLIB_SET="sillycam.la doofuscam.la" install
 
 Compiling all camlibs is sped up considerably on *N* CPU core
 computers using `make -j`*N*.

INSTALL

@@ -118,7 +118,7 @@ directories on your system.
 
 If you want to install only one or two specific camlibs, run something like
 
-    $ make CAMLIBS="canon.la ptp2.la" install-camlibs
+    $ make GP_CAMLIB_SET="canon.la ptp2.la" install-camlibs
 
 On Linux, the installed libtool .la files are not needed and can be removed.