fix(cmake): look for pkg-config when building grpc

[fntlnz]

Find pkg-config sooner than later, to avoid people to stumble into this error:

[ 40%] Performing patch step for 'grpc'
--2019-08-06 22:19:23--  https://download.sysdig.com/dependencies/grpc-1.8.1-Makefile.patch
Resolving download.sysdig.com (download.sysdig.com)... 54.230.117.108, 54.230.117.175, 54.230.117.33, ...
Connecting to download.sysdig.com (download.sysdig.com)|54.230.117.108|:443... connected.
HTTP request sent, awaiting response... 200 OK
Length: 673 [binary/octet-stream]
Saving to: 'grpc-1.8.1-Makefile.patch'

grpc-1.8.1-Makefile.patch  100%[======================================>]     673  --.-KB/s    in 0s

2019-08-06 22:19:24 (59.0 MB/s) - 'grpc-1.8.1-Makefile.patch' saved [673/673]

patching file Makefile
[ 40%] No update step for 'grpc'
[ 40%] No configure step for 'grpc'
[ 41%] Performing build step for 'grpc'

DEPENDENCY ERROR

The target you are trying to run requires protobuf 3.0.0+
Your system doesn't have it, and neither does the third_party directory.

Please consult INSTALL to get more information.

If you need information about why these tests failed, run:

  make run_dep_checks

Signed-off-by: Lorenzo Fontana lo@linux.com