cmake: unset forced CMAKE_C_STANDARD 90 (#822)

Added in cf80f2f4b5 (on 2016-08-14),
with the title "Basic dockerised test suite".

It's not clear why a C standard was explicitly set, but a side-effect
of this is that CMake-built binaries diverged from ones built with
autotools or GNU Make (using the same compiler and configuration).

Another issue is that this may introduce ABI incompatibility with
binaries built with a different C standard flag, e.g. the C compiler
default or one used for other components of a final app.

Seems unlikely, but if our tests require this option, we should set it
for the CI builds only?

CMakeLists.txt

@@ -44,8 +44,6 @@ project(libssh2 C)
 set(PROJECT_URL "https://www.libssh2.org/")
 set(PROJECT_DESCRIPTION "The SSH library")
 
-set(CMAKE_C_STANDARD 90)
-
 option(BUILD_SHARED_LIBS "Build Shared Libraries" OFF)
 
 # Parse version