Fix non-portable operator in shell conditional

Use = to compare strings with the test builtin command instead of ==,
which is non-standard and sometimes unsupported by stricter POSIX shells
(e.g., dash as /bin/sh).

Makefile.in

@@ -263,7 +263,7 @@ endif
 
 stamps/build-gcc-linux-stage1: $(GCC_SRCDIR) stamps/build-binutils-linux \
                                stamps/build-linux-headers
-	if test -f $</contrib/download_prerequisites && test "@NEED_GCC_EXTERNAL_LIBRARIES@" == "true"; then cd $< && ./contrib/download_prerequisites; fi
+	if test -f $</contrib/download_prerequisites && test "@NEED_GCC_EXTERNAL_LIBRARIES@" = "true"; then cd $< && ./contrib/download_prerequisites; fi
 	rm -rf $@ $(notdir $@)
 	mkdir $(notdir $@)
 	cd $(notdir $@) && $</configure \
@@ -425,7 +425,7 @@ stamps/build-gdb-newlib: $(GDB_SRCDIR)
 	mkdir -p $(dir $@) && touch $@
 
 stamps/build-gcc-newlib-stage1: $(GCC_SRCDIR) stamps/build-binutils-newlib
-	if test -f $</contrib/download_prerequisites && test "@NEED_GCC_EXTERNAL_LIBRARIES@" == "true"; then cd $< && ./contrib/download_prerequisites; fi
+	if test -f $</contrib/download_prerequisites && test "@NEED_GCC_EXTERNAL_LIBRARIES@" = "true"; then cd $< && ./contrib/download_prerequisites; fi
 	rm -rf $@ $(notdir $@)
 	mkdir $(notdir $@)
 	cd $(notdir $@) && $</configure \
@@ -581,7 +581,7 @@ stamps/build-binutils-musl: $(BINUTILS_SRCDIR) stamps/check-write-permission
 
 stamps/build-gcc-musl-stage1: $(GCC_SRCDIR) stamps/build-binutils-musl \
                                stamps/build-linux-headers
-	if test -f $</contrib/download_prerequisites && test "@NEED_GCC_EXTERNAL_LIBRARIES@" == "true"; then cd $< && ./contrib/download_prerequisites; fi
+	if test -f $</contrib/download_prerequisites && test "@NEED_GCC_EXTERNAL_LIBRARIES@" = "true"; then cd $< && ./contrib/download_prerequisites; fi
 	rm -rf $@ $(notdir $@)
 	mkdir $(notdir $@)
 	cd $(notdir $@) && $</configure \