[Linux] Define _GNU_SOURCE when building TSCclibc (#326)

Author: bnbarham

Package.swift

@@ -122,4 +122,10 @@ let package = Package(
       .linkedLibrary("Pathcch", .when(platforms: [.windows])),
     ]
   }
+#elseif os(Linux)
+  if let TSCclibc = package.targets.first(where: { $0.name == "TSCclibc" }) {
+    TSCclibc.cSettings = [
+      .define("_GNU_SOURCE", .when(platforms: [.linux])),
+    ]
+  }
 #endif

Sources/TSCclibc/CMakeLists.txt

@@ -10,6 +10,8 @@ add_library(TSCclibc STATIC
   libc.c process.c)
 target_include_directories(TSCclibc PUBLIC
   include)
+target_compile_definitions(TSCclibc PRIVATE
+  "$<$<PLATFORM_ID:Linux>:_GNU_SOURCE>")
 
 if(NOT BUILD_SHARED_LIBS)
   install(TARGETS TSCclibc