Fix pkg_config_entry when version number is not specified (#374)

When `gz_find_package` is called without a VERSION argument, the
`<package>_FIND_VERSION` variable is empty. But in most of our
`Find*.cmake` modules, we add a pkg-config entry that assumes the
version is there.

Example:
```
gz_pkg_config_entry(GzProtobuf "protobuf >= ${GzProtobuf_FIND_VERSION}")
```

The pkg-config entry will then be "protobuf >= ", which is invalid and
causes the following entry to be treated as a version number.

The fix here removes the operator if the version is empty.

Signed-off-by: Addisu Z. Taddese <addisu@openrobotics.org>

cmake/GzPkgConfig.cmake

@@ -205,8 +205,11 @@ macro(ign_pkg_config_entry package string)
   gz_pkg_config_entry(${package} ${string})
 endmacro()
 macro(gz_pkg_config_entry package string)
-
-  set(${package}_PKGCONFIG_ENTRY "${string}")
+  # The input string may contain an operator without a version,
+  # e.g "protobuf >= ". But this is not valid pkg-config syntax. This regex
+  # search/replace will remove the operator if the version is empty.
+  string(REGEX REPLACE " *[<>=]+ *$" "" entry ${string})
+  set(${package}_PKGCONFIG_ENTRY "${entry}")
   set(${package}_PKGCONFIG_TYPE PKGCONFIG_REQUIRES)
 
 endmacro()