use immediate evaluation for shell calls

This reworks the shell calls in the makefiles to use immediate
evaluation and should improve parallel building.

X-Gentoo-Bug: 289264
X-Gentoo-Bug-URL: http://bugs.gentoo.org/show_bug.cgi?id=289264

mk/git.mk

@@ -4,5 +4,5 @@ _GITVER_SH=	if git rev-parse --short HEAD >/dev/null 2>&1; then \
 		else \
 			echo ""; \
 		fi
-_GITVER!=	${_GITVER_SH}
-GITVER=		${_GITVER}$(shell ${_GITVER_SH})
+_GITVER:=	$(shell ${_GITVER_SH})
+GITVER=		${_GITVER}