configure.ac: make pkg-config optional when building with --disable-protoc

if pkg-config is installed, the libprotobuf-c .pc file will be
installed; if pkg-config is not installed, the .pc file won't be
installed.

this behavior only applies when we're building with ./configure
--disable-protoc, since pkg-config is required in order to detect the
protobuf dependency.

configure.ac

@@ -24,8 +24,6 @@ if test -n "$PKG_CONFIG"; then
     else
         PKG_INSTALLDIR
     fi
-else
-    AC_MSG_ERROR([pkg-config is required!])
 fi
 
 DX_DOXYGEN_FEATURE(OFF)