Xcode: Convert forced CMAKE_<LANG>_COMPILER to full path if possible

During cross-compiling the toolchain file may use CMakeForceCompiler to
force a compiler setting.  When using the Xcode generator try to convert
it to a full path by searching the PATH as is done for the Makefile
generators.

Modules/CMakeDetermineCCompiler.cmake

@@ -42,6 +42,7 @@ endif()
 if(${CMAKE_GENERATOR} MATCHES "Visual Studio")
 elseif("${CMAKE_GENERATOR}" MATCHES "Xcode")
   set(CMAKE_C_COMPILER_XCODE_TYPE sourcecode.c.c)
+  _cmake_find_compiler_path(C)
 else()
   if(NOT CMAKE_C_COMPILER)
     set(CMAKE_C_COMPILER_INIT NOTFOUND)

Modules/CMakeDetermineCXXCompiler.cmake

@@ -41,6 +41,7 @@ endif()
 if(${CMAKE_GENERATOR} MATCHES "Visual Studio")
 elseif("${CMAKE_GENERATOR}" MATCHES "Xcode")
   set(CMAKE_CXX_COMPILER_XCODE_TYPE sourcecode.cpp.cpp)
+  _cmake_find_compiler_path(CXX)
 else()
   if(NOT CMAKE_CXX_COMPILER)
     set(CMAKE_CXX_COMPILER_INIT NOTFOUND)

Modules/CMakeDetermineFortranCompiler.cmake

@@ -28,6 +28,7 @@ endif()
 if(${CMAKE_GENERATOR} MATCHES "Visual Studio")
 elseif("${CMAKE_GENERATOR}" MATCHES "Xcode")
   set(CMAKE_Fortran_COMPILER_XCODE_TYPE sourcecode.fortran.f90)
+  _cmake_find_compiler_path(Fortran)
 else()
   if(NOT CMAKE_Fortran_COMPILER)
     # prefer the environment variable CC