Revert "meson: use c_args in generator scripts (#10289)"

Revert "meson: use c_args in generator scripts (#10289)"

This reverts commit 56f56d5ad856d9bd1070693490b210e0a0ccde92.

This broke the compilation for coverity under travis. Our build script does
something like this:

$ CFLAGS='-D_Float128=long\ double -D_Float64=double -D_Float64x=long\ double -D_Float32=float -D_Float32x=double' meson cov-build -Dman=false
$ ninja -C build
...
[pid 27096] execve("/usr/bin/cc", ["/usr/bin/cc", "-D_Float128=long", "double", "-D_Float64=double", "-D_Float64x=long", "double", "-D_Float32=float", "-D_Float32x=double", "-E", "-dM", "-include", "linux/capability.h", "-include", "config.h", "-include", "../src/basic/missing.h", "-"], 0x55ab75ea4e80 /* 91 vars */) = 0
cc: error: double: No such file or directory
cc: error: double: No such file or directory
[pid 27096] +++ exited with 1 +++

I'm sure this could be fixed somehow, but since the original motivation for
56f56d5ad856d9bd1070693490b210e0a0ccde92 wasn't very strong, let's just revert
it as this seems to be the simplest solution.