build: fix typo related to finding ccache (#4833)

Wow, my fingers are so used to typing "exr", and I don't use the OS that
would make me ever type "exe", so...

Signed-off-by: Larry Gritz <lg@larrygritz.com>

Author: lgritz

src/cmake/compiler.cmake

@@ -239,10 +239,10 @@ if (CCACHE_EXE AND USE_CCACHE)
         message (STATUS "Ignoring ccache because clang + Qt + env CCACHE_CPP2 is not set")
     else ()
         if (NOT ${CXX_COMPILER_LAUNCHER} MATCHES "ccache")
-            set (CXX_COMPILER_LAUNCHER ${CCACHE_EXR} ${CXX_COMPILER_LAUNCHER})
+            set (CXX_COMPILER_LAUNCHER ${CCACHE_EXE} ${CXX_COMPILER_LAUNCHER})
         endif ()
         if (NOT ${C_COMPILER_LAUNCHER} MATCHES "ccache")
-            set (C_COMPILER_LAUNCHER ${CCACHE_EXR} ${C_COMPILER_LAUNCHER})
+            set (C_COMPILER_LAUNCHER ${CCACHE_EXE} ${C_COMPILER_LAUNCHER})
         endif ()
         message (STATUS "ccache enabled: ${CCACHE_EXE}")
     endif ()