RFR: 8013786: JDK-8013480 broke configure on solaris

[Erik Joelsson]

Unfortunately the change for bug JDK-8013480 broke the build on Solaris:

Configure no longer looks for cc and CC instead of gcc and g++ on 
solaris. This is caused by use of the macro AC_COMPILE_IFELSE in 
platform.m4, before the compilers have been initialized. We have 
implemented our platform dependent initialization of the compilers in 
toolchain.m4, which is executed after platform.m4. The macro 
AC_COMPILE_IFELSE runs AC_PROG_CC implicitly and sets CC to gcc, which 
is the default. Once done, AC_PROG_CC cannot be run again and so we get 
stuck with that value.

This patch moves these checks to toolchain.m4, after the compilers have 
been initialized. I'm currently running a jprt job verifying that this 
indeed works on all platforms.

http://cr.openjdk.java.net/~erikj/8013786/webrev.root.01/

/Erik