[lmi] ccache with autotools

[Greg Chicares]

AIUI, the recommended way to use ccache with autotools is to
override $CC and $CXX. However, in the cross-compilation case,
$CC and $CXX aren't necessarily specified. Is there a way to
accomplish this less bletcherously than as shown below?

diff --git a/install_wx.sh b/install_wx.sh
index 6129ff0e7..fef6add43 100755
--- a/install_wx.sh
+++ b/install_wx.sh
@@ -115,12 +115,21 @@ mkdir --parents "$build_dir"
 cd "$build_dir"
 printf 'Building %s with %s for %s.\n' "wx" "$LMI_COMPILER" "$LMI_TRIPLET"
 
+compiler_prefix="ccache "
+case "$LMI_TRIPLET" in
+    *-*-cygwin*|*-*-mingw*)
+        compiler_prefix="${compiler_prefix}x86_64-w64-mingw32-"
+        ;;
+esac
+
 case "$LMI_COMPILER" in
     (gcc)
         valid_math="-frounding-math -fsignaling-nans"
         # 'config_options' must not be double-quoted
         # shellcheck disable=SC2086
         "$wx_dir"/configure $config_options \
+                CC="${compiler_prefix}gcc" \
+               CXX="${compiler_prefix}g++" \
           CPPFLAGS="-I$prefix/include" \
             CFLAGS="$wx_cc_flags  $valid_math" \
           CXXFLAGS="$wx_cxx_flags $valid_math" \
@@ -132,8 +141,8 @@ case "$LMI_COMPILER" in
         # 'config_options' must not be double-quoted
         # shellcheck disable=SC2086
         "$wx_dir"/configure $config_options \
-                CC=clang \
-               CXX=clang++ \
+                CC="${compiler_prefix}clang" \
+               CXX="${compiler_prefix}clang++" \
           CPPFLAGS="-I$prefix/include" \
             CFLAGS="$wx_cc_flags  $valid_math" \
           CXXFLAGS="$wx_cxx_flags $valid_math -stdlib=libc++" \