Makefile: add NEEDS_LIBGEN to optionally add -lgen to compile arguments

Commit 003b33a8 recently added a call to basename().  On IRIX 6.5, this
function resides in libgen and -lgen is required for the linker.

Update configure.ac too.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Makefile

@@ -61,6 +61,8 @@ all::
 #
 # Define NO_LIBGEN_H if you don't have libgen.h.
 #
+# Define NEEDS_LIBGEN if your libgen needs -lgen when linking
+#
 # Define NO_SYS_SELECT_H if you don't have sys/select.h.
 #
 # Define NO_SYMLINK_HEAD if you never want .git/HEAD to be a symbolic link.
@@ -1019,6 +1021,9 @@ ifdef NEEDS_LIBICONV
 	endif
 	EXTLIBS += $(ICONV_LINK) -liconv
 endif
+ifdef NEEDS_LIBGEN
+	EXTLIBS += -lgen
+endif
 ifdef NEEDS_SOCKET
 	EXTLIBS += -lsocket
 endif

configure.ac

@@ -485,6 +485,12 @@ AC_CHECK_LIB([resolv], [hstrerror],
 AC_SUBST(NEEDS_RESOLV)
 test -n "$NEEDS_RESOLV" && LIBS="$LIBS -lresolv"
 
+AC_CHECK_LIB([gen], [basename],
+[NEEDS_LIBGEN=],
+[NEEDS_LIBGEN=YesPlease])
+AC_SUBST(NEEDS_LIBGEN)
+test -n "$NEEDS_LIBGEN" && LIBS="$LIBS -lgen"
+
 ## Checks for header files.
 AC_MSG_NOTICE([CHECKS for header files])
 #