VS11: Fix VSExternalInclude test

Set CMAKE_SUPPRESS_REGENERATION in the Lib1 and Lib2 projects so that
their .vcxproj files do not contain references to ZERO_CHECK.  Such
references do not make sense when using the files in another .sln file.
This does not reduce the effectiveness of the test because real projects
that use include_external_msproject will have their own .vcxproj files not
generated by CMake anyway.

Tests/VSExternalInclude/Lib1/CMakeLists.txt

@@ -1,3 +1,4 @@
+set(CMAKE_SUPPRESS_REGENERATION 1)
 project(LIB1)
 
 set(SOURCES lib1.cpp)

Tests/VSExternalInclude/Lib2/CMakeLists.txt

@@ -1,3 +1,4 @@
+set(CMAKE_SUPPRESS_REGENERATION 1)
 project(VSEXTERNAL_LIB2)
 
 include_directories(${VSEXTERNAL_LIB2_SOURCE_DIR}/../Lib1)