protobuf: make python-protobuf dependency optional and default to off

A dependency on python-protobuf was added in commit
5f6fcfd362 for the sole purpose of
providing a ptest, however python-protobuf is in meta-python and thus
this means that meta-oe would depend on meta-python by default (assuming
your distro enables ptest by default), and we don't want that - meta-oe
isn't supposed to depend upon any layer other than openembedded-core.
Luckily we can still have a ptest even without the python support, so
add a PACKAGECONFIG and leave it disabled by default.

Note: the PACKAGECONFIG here is not particularly useful since it's only
about what goes into the -ptest package and thus also the dependency. I
contemplated just using LANG_SUPPORT instead, but PACKAGECONFIG does
have the advantage that it's introspectable and fairly well understood
so in the end I went with it.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>

meta-oe/recipes-devtools/protobuf/protobuf_3.5.1.bb

@@ -12,7 +12,7 @@ DEPENDS = "zlib"
 DEPENDS_append_class-target  = " protobuf-native"
 RDEPENDS_${PN}-compiler = "${PN}"
 RDEPENDS_${PN}-dev += "${PN}-compiler"
-RDEPENDS_${PN}-ptest = "bash python-protobuf"
+RDEPENDS_${PN}-ptest = "bash ${@bb.utils.contains('PACKAGECONFIG', 'python', 'python-protobuf', '', d)}"
 
 LIC_FILES_CHKSUM = "file://LICENSE;md5=35953c752efc9299b184f91bef540095"
 
@@ -24,13 +24,16 @@ SRC_URI = "git://github.com/google/protobuf.git;branch=3.5.x \
 	   file://run-ptest \
           "
 
+PACKAGECONFIG ??= ""
+PACKAGECONFIG[python] = ",,"
+
 EXTRA_OECONF += " --with-protoc=echo"
 
 inherit autotools-brokensep pkgconfig ptest
 
 S = "${WORKDIR}/git"
 TEST_SRC_DIR="examples"
-LANG_SUPPORT="cpp python"
+LANG_SUPPORT = "cpp ${@bb.utils.contains('PACKAGECONFIG', 'python', 'python', '', d)}"
 
 do_compile_ptest() {
 	# Modify makefile to use the cross-compiler