Disable pkg-config-detected externals (#671)

Proper use of these packages would require calling `pkg-config` to set up
environment, which Alire does not do (yet?)

Co-authored-by: GHA <actions@github.com>

index/li/libgmp/libgmp-external.toml

@@ -15,3 +15,7 @@ msys2 = ["mingw-w64-x86_64-gmp"]
 kind = "version-output"
 version-command = [ "pkg-config", "--modversion", "gmp" ]
 version-regexp = "([\\d\\.]+)"
+
+# pkg-config requires further steps to make packages findable by
+# dependencies that Alire does not yet support
+available = false

index/li/libgnutls/libgnutls-external.toml

@@ -14,3 +14,7 @@ kind = "system"
 kind = "version-output"
 version-command = [ "pkg-config", "--modversion", "gnutls" ]
 version-regexp = "([\\d\\.]+)"
+
+# pkg-config requires further steps to make packages findable by
+# dependencies that Alire does not yet support
+available = false

index/li/libgtk3/libgtk3-external.toml

@@ -16,3 +16,6 @@ msys2 = ["mingw-w64-x86_64-gtk3"]
 kind = "version-output"
 version-command = [ "pkg-config", "--modversion", "gtk+-3.0" ]
 version-regexp = "([\\d\\.]+)"
+# pkg-config requires further steps to make packages findable by
+# dependencies that Alire does not yet support
+available = false