cmake: Do not depend on a C compiler to be present by explicitly stating that this is a C++ project.

CMake defaults to C *and* C++ toolchain, in case nothing has been specified.
This means that cmake always checks for both, which is more than needed.

This PR cuts off C toolchain requirement from /CMakeLists.txt and ensures that we
don't pass along any `..._C_...` variables in EthCompilerSettings.cake nor jsoncpp.cmake.

CMakeLists.txt

@@ -9,7 +9,7 @@ eth_policy()
 
 # project name and version should be set after cmake_policy CMP0048
 set(PROJECT_VERSION "0.5.3")
-project(solidity VERSION ${PROJECT_VERSION})
+project(solidity VERSION ${PROJECT_VERSION} LANGUAGES CXX)
 
 option(LLL "Build LLL" OFF)
 option(SOLC_LINK_STATIC "Link solc executable statically on supported platforms" OFF)

cmake/EthCompilerSettings.cmake

@@ -166,9 +166,8 @@ option(USE_CVC4 "Allow compiling with CVC4 SMT solver integration" ON)
 if (("${CMAKE_CXX_COMPILER_ID}" MATCHES "GNU") OR ("${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang"))
 	option(USE_LD_GOLD "Use GNU gold linker" ON)
 	if (USE_LD_GOLD)
-		execute_process(COMMAND ${CMAKE_C_COMPILER} -fuse-ld=gold -Wl,--version ERROR_QUIET OUTPUT_VARIABLE LD_VERSION)
+		execute_process(COMMAND ${CMAKE_CXX_COMPILER} -fuse-ld=gold -Wl,--version ERROR_QUIET OUTPUT_VARIABLE LD_VERSION)
 		if ("${LD_VERSION}" MATCHES "GNU gold")
-			set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -fuse-ld=gold")
 			set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -fuse-ld=gold")
 		endif ()
 	endif ()

cmake/jsoncpp.cmake

@@ -35,7 +35,6 @@ ExternalProject_Add(jsoncpp-project
     URL_HASH SHA256=c49deac9e0933bcb7044f08516861a2d560988540b23de2ac1ad443b219afdb6
     CMAKE_COMMAND ${JSONCPP_CMAKE_COMMAND}
     CMAKE_ARGS -DCMAKE_INSTALL_PREFIX=<INSTALL_DIR>
-               -DCMAKE_C_COMPILER=${CMAKE_C_COMPILER}
                -DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}
                -DCMAKE_INSTALL_LIBDIR=lib
                # Build static lib but suitable to be included in a shared lib.