libuv: fix C standard selection on Solaris 5.10 and 5.11
On Solaris 5.10 we need to build libuv as C90. Make it do so even when using SunPro 5.14 that defaults to C11.
On Solaris 5.11 we need to build libuv as C99+. Make it do so even when using SunPro 5.13 or earlier that default to C90.