Manylinux toolchain: set MANYLINUX1/MANYLINUX2010 to TRUE

So that one can do `if (MANYLINUX1)` in CMake, just like it
is commonly done with e.g. `if (IOS)` or `if (MSVC)`.

manylinux1-x64/Toolchain.cmake

@@ -2,6 +2,8 @@ set(CMAKE_SYSTEM_NAME Linux)
 set(CMAKE_SYSTEM_VERSION 1)
 set(CMAKE_SYSTEM_PROCESSOR x86_64)
 
+set(MANYLINUX1 TRUE)
+
 set(cross_triple "x86_64-linux-gnu")
 
 set(CMAKE_C_COMPILER /opt/rh/devtoolset-2/root/usr/bin/gcc)