Makefile: run the cd commands in a subshell

In bmake, if the directory is changed (with cd or anything else), bmake
won't return to the "root directory" on the next command (in the same
Makefile rule). This commit runs the cd command in a subshell so it
would work in bmake.

Closes #5073

Makefile.am

@@ -289,15 +289,15 @@ cygwinbin:
 
 # We extend the standard install with a custom hook:
 install-data-hook:
-	cd include && $(MAKE) install
-	cd docs && $(MAKE) install
-	cd docs/libcurl && $(MAKE) install
+	(cd include && $(MAKE) install)
+	(cd docs && $(MAKE) install)
+	(cd docs/libcurl && $(MAKE) install)
 
 # We extend the standard uninstall with a custom hook:
 uninstall-hook:
-	cd include && $(MAKE) uninstall
-	cd docs && $(MAKE) uninstall
-	cd docs/libcurl && $(MAKE) uninstall
+	(cd include && $(MAKE) uninstall)
+	(cd docs && $(MAKE) uninstall)
+	(cd docs/libcurl && $(MAKE) uninstall)
 
 ca-bundle: lib/mk-ca-bundle.pl
 	@echo "generating a fresh ca-bundle.crt"
@@ -308,11 +308,11 @@ ca-firefox: lib/firefox-db2pem.sh
 	./lib/firefox-db2pem.sh lib/ca-bundle.crt
 
 checksrc:
-	cd lib && $(MAKE) checksrc
-	cd src && $(MAKE) checksrc
-	cd tests && $(MAKE) checksrc
-	cd include/curl && $(MAKE) checksrc
-	cd docs/examples && $(MAKE) checksrc
+	(cd lib && $(MAKE) checksrc)
+	(cd src && $(MAKE) checksrc)
+	(cd tests && $(MAKE) checksrc)
+	(cd include/curl && $(MAKE) checksrc)
+	(cd docs/examples && $(MAKE) checksrc)
 
 .PHONY: vc-ide