Respect CC, *FLAGS.  Add datarootdir for autoconf-2.60 warnings.

--- ftjam-2.5.3rc2-orig/builds/unix/Makefile.in
+++ ftjam-2.5.3rc2/builds/unix/Makefile.in
@@ -37,6 +37,7 @@ exec_prefix  := @exec_prefix@
 libdir       := @libdir@
 bindir       := @bindir@
 includedir   := @includedir@
+datarootdir  := @datarootdir@
 datadir      := @datadir@
 
 version_info := @version_info@
--- ftjam-2.5.3rc2-orig/jambase.c
+++ ftjam-2.5.3rc2/jambase.c
@@ -652,21 +652,21 @@ const char *jambase[] = {
 "\n",
 "AR      ?= ar ru ;\n",
 "AS      ?= as ;\n",
-"ASFLAGS     ?= ;\n",
+"ASFLAGS     += $(ASFLAGS) ;\n",
 "AWK     ?= awk ;\n",
 "BINDIR      ?= /usr/local/bin ;\n",
-"C++     ?= cc ;\n",
-"C++FLAGS    ?= ;\n",
-"CC      ?= cc ;\n",
-"CCFLAGS     ?= ;\n",
+"C++     ?= $(CXX) ;\n",
+"C++FLAGS    += $(CXXFLAGS) ;\n",
+"CC      ?= $(CC) ;\n",
+"CCFLAGS     += $(CFLAGS) ;\n",
 "CP      ?= cp -f ;\n",
 "CRELIB      ?= ;\n",
 "DOT     ?= . ;\n",
 "DOTDOT      ?= .. ;\n",
 "EXEMODE     ?= 711 ;\n",
 "FILEMODE    ?= 644 ;\n",
-"FORTRAN     ?= f77 ;\n",
-"FORTRANFLAGS    ?= ;\n",
+"FORTRAN     ?= $(F77) ;\n",
+"FORTRANFLAGS    += $(FFLAGS) ;\n",
 "HDRS        ?= ;\n",
 "INSTALLGRIST    ?= installed ;\n",
 "JAMFILE     ?= Jamfile ;\n",
@@ -674,13 +674,13 @@ const char *jambase[] = {
 "LEX     ?= ;\n",
 "LIBDIR      ?= /usr/local/lib ;\n",
 "LINK        ?= $(CC) ;\n",
-"LINKFLAGS   ?= ;\n",
+"LINKFLAGS   += $(LDFLAGS) ;\n",
 "LINKLIBS    ?= ;\n",
 "LN      ?= ln ;\n",
 "MANDIR      ?= /usr/local/man ;\n",
 "MKDIR       ?= mkdir ;\n",
 "MV      ?= mv -f ;\n",
-"OPTIM       ?= ;\n",
+"OPTIM       = ;\n",
 "RCP     ?= rcp ;\n",
 "RM      ?= rm -f ;\n",
 "RMDIR       ?= $(RM) ;\n",