Apply flags for cross-compilation

This MR applies compiler and linker flags for cross-compilation toolchain.

Benefits:

• We get optimized and hardened Windows binaries instead of default -O0 ones. Basically do the same thing as with Linux binaries.
• The package size is significantly reduced (from current ~1.3 GiB to ~600 MiB), because optimized binaries take less space.

Existing flags require some tweaking though:

• -Werror=format-security compiler flag can't be applied as some Windows code has security warnings.
• -Wl,-z linker flags are not supported by MinGW ld.

PKGBUILD

@@ -97,6 +97,11 @@ build() {
   # Doesn't compile without remove these flags as of 4.10
   export CFLAGS="$CFLAGS -ffat-lto-objects"
 
+  # Apply flags for cross-compilation
+  export CROSSCFLAGS="${CFLAGS/-Werror=format-security/}"
+  export CROSSCXXFLAGS="${CXXFLAGS/-Werror=format-security/}"
+  export CROSSLDFLAGS="${LDFLAGS//-Wl,-z*([^[:space:]])/}"
+
   msg2 "Building Wine-64..."
   mkdir "$pkgname-64-build"
   cd "$pkgname-64-build"