Fix for explicit module build

Disambiguate the modf call to the C version and add a header search
path.

Author: hjyamauchi

Package.swift

@@ -219,6 +219,7 @@ let SwiftFirebase =
                                 cxxSettings: [
                                   .define("INTERNAL_EXPERIMENTAL"),
                                   .define("_CRT_SECURE_NO_WARNINGS", .when(platforms: [.windows])),
+                                  .headerSearchPath("../../third_party/firebase-development/usr/include"),
                                 ],
                                 swiftSettings: [
                                   .interoperabilityMode(.Cxx),

Sources/FirebaseFirestore/Timestamp+Swift.swift

@@ -50,7 +50,17 @@ extension Timestamp: Codable {
 extension Date {
   internal func firestoreTimestamp() -> Timestamp {
     var secondsDouble: Double = 0.0
-    var fraction = modf(timeIntervalSince1970, &secondsDouble)
+#if canImport(Darwin.C)
+    var fraction = Darwin.C.modf(timeIntervalSince1970, &secondsDouble)
+#elseif canImport(SwiftGlibc)
+    var fraction = SwiftGlibc.modf(timeIntervalSince1970, &secondsDouble)
+#elseif canImport(ucrt)
+    var fraction = ucrt.modf(timeIntervalSince1970, &secondsDouble)
+#elseif canImport(SwiftAndroid)
+    var fraction = SwiftAndroid.modf(timeIntervalSince1970, &secondsDouble)
+#else
+    #error("Unsupported platform")
+#endif
 
     // Re-implementation of https://github.com/firebase/firebase-ios-sdk/blob/master/Firestore/Source/API/FIRTimestamp.m#L50
     if (fraction < 0) {