package/exim: fix build with ccache

When using ccache to build the exim package, the HOSTCC value contains
spaces, that are incorrectly interpreted by exim's Makefilei, which uses
the first word of ${CC} to test compiler options. This breaks the build
with "unrecognized option" ccache errors.

Fix that by wrapping the HOSTCC variable in double quotes, as it is done
for other variables that follow.

Signed-off-by: Alejandro González <alejandro.gonzalez.correo@gmail.com>
[yann.morin.1998@free.fr: slight rewording of commit log]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
(cherry picked from commit a9486e337ac7b32a58c31618a070c1b9f9ca9506)
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/exim/exim.mk

@@ -123,8 +123,8 @@ endif
 define EXIM_BUILD_CMDS
 	$(TARGET_MAKE_ENV) build=br $(MAKE) -C $(@D) makefile
 	$(HOST_MAKE_ENV) $(MAKE) -C $(@D)/build-br macro_predef \
-		CC=$(HOSTCC) \
-		LNCC=$(HOSTCC) \
+		CC="$(HOSTCC)" \
+		LNCC="$(HOSTCC)" \
 		CFLAGS="-std=c99 $(HOST_CFLAGS)" \
 		LFLAGS="-fPIC $(HOST_LDFLAGS)"
 	$(TARGET_MAKE_ENV) build=br $(MAKE) -C $(@D) $(EXIM_STATIC_FLAGS) \