package/pkg-meson.mk: fix ccache auto-detection avoidance

Commit f4a61d1ae2 (package/pkg-meson.mk avoid host ccache detection)
forced the host C and C++ compilers so that meson does not try to
autodetect ccache, and instead relies on what we provide.

However, this incorrectly used single-expansion of variables in a
package infra.

For traditional builds, this is OK, because the value does not change
across packages.

However, for builds with per-package directories, this value only refers
to the generic path, which ill not exist until the end of the build when
all packages are aggregated in the host-finalize step.

Fix that by postponing the variable evaluation like all the others.

Reported-by: Xogium on IRC
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

package/pkg-meson.mk

@@ -82,8 +82,8 @@ define $(2)_CONFIGURE_CMDS
 	    package/meson/cross-compilation.conf.in \
 	    > $$($$(PKG)_SRCDIR)/build/cross-compilation.conf
 	PATH=$$(BR_PATH) \
-	CC_FOR_BUILD="$(HOSTCC)" \
-	CXX_FOR_BUILD="$(HOSTCC)" \
+	CC_FOR_BUILD="$$(HOSTCC)" \
+	CXX_FOR_BUILD="$$(HOSTCC)" \
 	$$($$(PKG)_CONF_ENV) \
 	$$(MESON) \
 		--prefix=/usr \