Fixed install directory issue

Now you can specify LIBDIR and INCLUDEDIR along with
DESTDIR while installing. The library goes into
/usr/local by default, but the target directory can
change (for instance when installing from a package).

Makefile

@@ -18,8 +18,11 @@ REAL_NAME=$(SONAME).$(MINOR_VERSION).$(RELEASE)
 STATIC_NAME=$(NAME).a
 
 DESTDIR=
-HEADERS_INSTALL_PATH=$(DESTDIR)/usr/local/include
-LIB_INSTALL_PATH=$(DESTDIR)/usr/local/lib
+INCLUDEDIR=/usr/local/include
+LIBDIR=/usr/local/lib
+
+HEADERS_INSTALL_PATH=$(DESTDIR)$(INCLUDEDIR)
+LIB_INSTALL_PATH=$(DESTDIR)$(LIBDIR)
 
 HEADERS_DIR=include/
 DOCUMENTATION_DIR=doc/