cmake: Fix Android build for r25 NDK

Currently with the build instructions provided in README.md
the build will fail.

In the r25 NDK the CMake toolchain defaults to the legacy path,
due to a bug in the current implementation.

https://github.com/android/ndk/issues/323

CMakeLists.txt

@@ -126,7 +126,7 @@ if(WIN32)
         include(ChooseMSVCCRT.cmake)
     endif()
     add_definitions(-DGLSLANG_OSINCLUDE_WIN32)
-elseif(UNIX)
+elseif(UNIX OR ANDROID)
     add_definitions(-DGLSLANG_OSINCLUDE_UNIX)
 else()
     message("unknown platform")

glslang/CMakeLists.txt

@@ -33,7 +33,7 @@
 
 if(WIN32)
     add_subdirectory(OSDependent/Windows)
-elseif(UNIX OR "${CMAKE_SYSTEM_NAME}" STREQUAL "Fuchsia")
+elseif(UNIX OR "${CMAKE_SYSTEM_NAME}" STREQUAL "Fuchsia" OR ANDROID)
     add_subdirectory(OSDependent/Unix)
 else()
     message("unknown platform")