[source] rules.mk: make PKG_CONFIG_DEPENDS properly track string values

[LEDE Commits]

jow pushed a commit to source.git, branch lede-17.01:
https://git.lede-project.org/4af145ea670d5cc0513948c9265f289b0a2fa844

commit 4af145ea670d5cc0513948c9265f289b0a2fa844
Author: Matthias Schiffer <mschiffer at universe-factory.net>
AuthorDate: Fri Feb 24 12:16:33 2017 +0100

    rules.mk: make PKG_CONFIG_DEPENDS properly track string values
    
    The confvar macro is adjusted to not only consider if a variable has a
    value or not, but also the value itself. Instead of creating a string of
    'y' and 'n' characters, all variable names and values are concatenated
    and hashed.
    
    Signed-off-by: Matthias Schiffer <mschiffer at universe-factory.net>
    (cherry picked from commit 5ef0854b1109ba2dbd1dd3d9f87ce5801002d0ba)
---
 rules.mk | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/rules.mk b/rules.mk
index 04fd936..d58f8d9 100644
--- a/rules.mk
+++ b/rules.mk
@@ -29,7 +29,7 @@ empty:=
 space:= $(empty) $(empty)
 comma:=,
 merge=$(subst $(space),,$(1))
-confvar=$(call merge,$(foreach v,$(1),$(if $($(v)),y,n)))
+confvar=$(shell echo '$(foreach v,$(1),$(v)=$(subst ','\'',$($(v))))' | $(STAGING_DIR_HOST)/bin/mkhash md5)
 strip_last=$(patsubst %.$(lastword $(subst .,$(space),$(1))),%,$(1))
 
 paren_left = (