[build] Fix value of CMAKE_C{,XX}_ARG when CC_ARG doesn't have args

[giordano]

The problem was that cut by default prints the entire line if the delimiter doesn't appear, unless the option -s is used, which means that if CC_ARG contains only gcc then CMAKE_CC_ARG also contains gcc instead of being empty.

Before the PR:

$ make -C deps/ print-CC_ARG print-CMAKE_CC_ARG print-CMAKE_COMMON USECCACHE=1
make: Entering directory '/home/mose/repo/julia/deps'
CC_ARG=gcc
CMAKE_CC_ARG=gcc
CMAKE_COMMON=-DCMAKE_INSTALL_PREFIX:PATH=/home/mose/repo/julia/usr -DCMAKE_PREFIX_PATH=/home/mose/repo/julia/usr -DLIB_INSTALL_DIR=/home/mose/repo/julia/usr/lib -DCMAKE_INSTALL_LIBDIR=/home/mose/repo/julia/usr/lib -DCMAKE_C_COMPILER_LAUNCHER=ccache -DCMAKE_CXX_COMPILER_LAUNCHER=ccache -DCMAKE_C_COMPILER="$(which gcc)" -DCMAKE_C_COMPILER_ARG1="gcc " -DCMAKE_CXX_COMPILER="$(which g++)" -DCMAKE_CXX_COMPILER_ARG1="g++ " -DCMAKE_LINKER="$(which ld)" -DCMAKE_AR="$(which ar)" -DCMAKE_RANLIB="$(which ranlib)"
make: Leaving directory '/home/mose/repo/julia/deps'

After the PR

$ make -C deps/ print-CC_ARG print-CMAKE_CC_ARG print-CMAKE_COMMON USECCACHE=1
make: Entering directory '/home/mose/repo/julia/deps'
CC_ARG=gcc
CMAKE_CC_ARG=
CMAKE_COMMON=-DCMAKE_INSTALL_PREFIX:PATH=/home/mose/repo/julia/usr -DCMAKE_PREFIX_PATH=/home/mose/repo/julia/usr -DLIB_INSTALL_DIR=/home/mose/repo/julia/usr/lib -DCMAKE_INSTALL_LIBDIR=/home/mose/repo/julia/usr/lib -DCMAKE_C_COMPILER_LAUNCHER=ccache -DCMAKE_CXX_COMPILER_LAUNCHER=ccache -DCMAKE_C_COMPILER="$(which gcc)" -DCMAKE_CXX_COMPILER="$(which g++)" -DCMAKE_LINKER="$(which ld)" -DCMAKE_AR="$(which ar)" -DCMAKE_RANLIB="$(which ranlib)"
make: Leaving directory '/home/mose/repo/julia/deps'

as intended.

What I don't understand is why we use CMAKE_C_COMPILER_ARG1 in the first place, which is an undocumented hack.

[giordano]

What I don't understand is why we use CMAKE_C_COMPILER_ARG1 in the first place, which is an undocumented hack.

It was introduced in #8906, funnily enough to make USECCACHE=1 work, which is what was breaking for me here, because we ended up passing the wrong -DCMAKE_C_COMPILER_ARG1="gcc ".