README: Add documentation for BUILD_DYNAMIC_LIB

Document the newly introduced BUILD_DYNAMIC_LIB flag and clarify the
use of the other variables USE_STATIC_LIB and BUILD_STATIC_LIB.

[JD: Edit]

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
Signed-off-by: Jean Delvare <jdelvare@suse.de>

README

@@ -63,8 +63,18 @@ tools, and "make install" to install them. You also can use "make uninstall"
 to remove all the files you installed. By default, files are installed in
 /usr/local but you can change the location by editing the Makefile file and
 setting prefix to wherever you want. You may change the C compiler and the
-compilation flags as well, and also decide whether to build the static
-library or not.
+compilation flags as well, and also decide which flavors of the library
+will be built.
+
+In cases where you need to compile only the static or dynamic variant of
+the library, you can use BUILD_STATIC_LIB and BUILD_DYNAMIC_LIB at compile
+time:
+  $ make BUILD_DYNAMIC_LIB=1 BUILD_STATIC_LIB=0
+If you disable both, the compilation will abort with an error message.
+
+If you want to compile the tools against the static version of the library
+you can do:
+  $ make USE_STATIC_LIB=1
 
 Optionally, you can run "make strip" prior to "make install" if you want
 smaller binaries. However, be aware that this will prevent any further