mklib build appears to fail when CCS is installed in a path with whitespaces. In the example below, CCS is installed in "E:\Program Files\Texas Instruments":
warning #10366-D: automatic library build: using library "E:\Program Files\Texas Instruments\ccs\tools\compiler\ti-cgt-msp430_20.2.1.LTS\lib\rts430x_lc_ld_eabi.lib" for the first time, so it must be built. This may take a few minutes.
'e:/program' is not recognized as an internal or external command, operable program or batch file.
>> ERROR: mklib: gmake error during rts430x_lc_ld_eabi.lib build