Clarify Android Windows build instructions

(must add .exe to CMAKE_C_COMPILER)

Addresses a concern raised in #245, #260
diff --git a/BUILDING.md b/BUILDING.md
index 2ebc6e0..b11ab8b 100644
--- a/BUILDING.md
+++ b/BUILDING.md
@@ -682,6 +682,8 @@
 `-DCMAKE_POSITION_INDEPENDENT_CODE=1` from the CMake arguments and `-pie` from
 `LDFLAGS`.
 
+If building on Windows, add `.exe` to the end of `CMAKE_C_COMPILER`.
+
 
 Advanced CMake Options
 ----------------------