normalize single-token compiler vars in tbb build

when a Makevars sets e.g. 'CXX=$(CCACHE) g++' with CCACHE empty, the
value expands to ' g++' with a leading space. splitCompilerVar() would
tokenize this to a single token and return early without re-setting the
variable, leaving the leading space in place and forwarding an invalid
'-DCMAKE_CXX_COMPILER= g++' to cmake, breaking the bundled tbb build.

re-set the compiler from the parsed tokens even when there are no
trailing flags; scan() has already stripped surrounding whitespace.

Author: kevinushey

src/install.libs.R

@@ -340,9 +340,13 @@ splitCompilerVar <- function(compilerVar, flagsVar) {
       return(FALSE)
 
    tokens <- scan(text = compiler, what = character(), quiet = TRUE)
-   if (length(tokens) < 2L)
+   if (length(tokens) == 0L)
       return(FALSE)
 
+   # always re-set the compiler from the parsed tokens, even when there are
+   # no trailing flags: scan() strips surrounding whitespace, so this also
+   # normalizes values like ' g++' (produced when e.g. '$(CCACHE) g++'
+   # expands with an empty CCACHE), which CMake would otherwise reject
    setenv(compilerVar, tokens[[1L]])
 
    oldFlags <- Sys.getenv(flagsVar)