www/py-django50: unbreak build with HTMLDOCS on (+)

docs/Makefile composed with gmake instructions, so call gmake as build dependency when needed

Reported by:	poudriere failure
Approved by:	portmgr blanket (trivial build fix)

www/py-django50/Makefile

@@ -48,6 +48,7 @@ HTMLDOCS_DESC=	Install HTML documentation (requires Sphinx)
 
 HTMLDOCS_BUILD_DEPENDS=	${PYTHON_PKGNAMEPREFIX}sphinx>=0,1:textproc/py-sphinx@${PY_FLAVOR}
 HTMLDOCS_IMPLIES=	DOCS
+HTMLDOCS_USES=		gmake
 PGSQL_RUN_DEPENDS=	${PYTHON_PKGNAMEPREFIX}psycopg2>0:databases/py-psycopg2@${PY_FLAVOR}
 MYSQL_RUN_DEPENDS=	${PYTHON_PKGNAMEPREFIX}mysqlclient>=1.3.13:databases/py-mysqlclient@${PY_FLAVOR}
 SQLITE_RUN_DEPENDS=	${PYTHON_PKGNAMEPREFIX}sqlite3>0:databases/py-sqlite3@${PY_FLAVOR}
@@ -59,7 +60,7 @@ IGNORE=		requires Python 3.10 or later
 .endif
 
 post-build-HTMLDOCS-on:
-	cd ${WRKSRC}/docs && ${MAKE} html SPHINXBUILD=sphinx-build-${PYTHON_VER}
+	cd ${WRKSRC}/docs && ${GMAKE} html SPHINXBUILD=sphinx-build-${PYTHON_VER}
 
 post-install:
 	${INSTALL_MAN} ${WRKSRC}/docs/man/* ${STAGEDIR}/${PREFIX}/share/man/man1