[mono-runtimes] Add and use $(HostCc), $(HostCxx)

`clang` isn't the C compiler everywhere, and `clang++` isn't the
C++ everywhere. For example, Ubuntu provides `clang-3.5`.

Allow the host-native C and C++ compilers to be specified/overridden
via the new $(HostCc) and $(HostCxx) MSBuild properties.

Configuration.Override.props.in

@@ -6,6 +6,10 @@
     <!-- The Xamarin.Android $(TargetFrameworkVersion) value that corresponds to $(AndroidApiLevel) -->
     <AndroidFrameworkVersion>v6.0</AndroidFrameworkVersion>
 
+    <!-- C and C++ compilers to emit host-native binaries -->
+    <HostCc>clang</HostCc>
+    <HostCxx>clang++</HostCxx>
+
     <!-- These must be FULL PATHS -->
     <AndroidToolchainCacheDirectory>$(HOME)\android-archives</AndroidToolchainCacheDirectory>
     <AndroidToolchainDirectory>$(HOME)\android-toolchain</AndroidToolchainDirectory>

Configuration.props

@@ -5,6 +5,8 @@
     <HostOS Condition=" '$(HostOS)' == '' And '$(OS)' == 'Windows_NT' ">Windows</HostOS>
     <HostOS Condition=" '$(HostOS)' == '' And '$(OS)' == 'Unix' And Exists ('/Applications') ">Darwin</HostOS>
     <HostOS Condition=" '$(HostOS)' == '' And '$(OS)' == 'Unix' ">Linux</HostOS>
+    <HostCc Condition=" '$(HostCc)' == '' ">clang</HostCc>
+    <HostCxx Condition=" '$(HostCc)' == '' ">clang++</HostCxx>
     <ManagedRuntime Condition=" '$(ManagedRuntime)' == '' And '$(OS)' != 'Windows_NT' ">mono</ManagedRuntime>
     <HOME Condition=" '$(HOME)' == '' ">$(HOMEDRIVE)$(HOMEPATH)</HOME>
     <AndroidApiLevel Condition=" '$(AndroidApiLevel)' == '' ">23</AndroidApiLevel>

README.md

@@ -33,6 +33,8 @@ Overridable MSBuild properties include:
 * `$(AndroidToolchainDirectory)`: The directory to install the downloaded
     Android NDK and SDK files. This value defaults to
     `$(HOME)\android-toolchain`.
+* `$(HostCc)`, `$(HostCxx)`: The C and C++ compilers to use to generate
+    host-native binaries.
 
 # Build Requirements
 

build-tools/mono-runtimes/mono-runtimes.projitems

@@ -22,9 +22,9 @@
     <_MonoRuntime Include="host">
       <Ar>ar</Ar>
       <As>as</As>
-      <Cc>clang</Cc>
+      <Cc>$(HostCc)</Cc>
       <CFlags>$(_CommonCFlags) -mmacosx-version-min=10.9</CFlags>
-      <Cxx>clang++</Cxx>
+      <Cxx>$(HostCxx)</Cxx>
       <CxxCpp>cpp</CxxCpp>
       <CxxFlags></CxxFlags>
       <Ld>ld</Ld>