meson: Don't use paths with backslashes in defines

[nirbheek]

Breaks the build on MSYS2 when building inside a directory with a component starting with U, like C:\Users:

FAILED: tests/test-is-public-all.exe.p/test-is-public-all.c.obj
"cc"  "-Itests\test-is-public-all.exe.p" "-Itests" "-I..\tests" "-I." "-I.." "-Iinclude" "-I..\include" "-fdiagnostics-color=always" "-D_FILE_OFFSET_BITS=64" "-Wall" "-Winvalid-pch" "-O0" "-g" "-DHAVE_CONFIG_H" "-DSRCDIR=\"C:\\Users\\nirbheek\\tmp\\gstreamer\\subprojects\\libpsl\\tests\"" "-DPSL_FILE=\"C:/Users/nirbheek/tmp/gstreamer/subprojects/libpsl/list/public_suffix_list.dat\"" "-DPSL_TESTFILE=\"C:/Users/nirbheek/tmp/gstreamer/subprojects/libpsl/list/tests/tests.txt\"" "-DPSL_DAFSA=\"C:\\Users\\nirbheek\\tmp\\gstreamer\\subprojects\\libpsl\\builddir\\tests\\psl.dafsa\"" "-DPSL_ASCII_DAFSA=\"C:\\Users\\nirbheek\\tmp\\gstreamer\\subprojects\\libpsl\\builddir\\tests\\psl_ascii.dafsa\"" -MD -MQ tests/test-is-public-all.exe.p/test-is-public-all.c.obj -MF "tests\test-is-public-all.exe.p\test-is-public-all.c.obj.d" -o tests/test-is-public-all.exe.p/test-is-public-all.c.obj "-c" ../tests/test-is-public-all.c
../tests/test-is-public-all.c: In function 'test_psl':
../tests/test-is-public-all.c:149:45: error: incomplete universal character name \U
  149 |         if (!(psl3 = psl_load_file(PSL_DAFSA))) {
      |                                             ^
../tests/test-is-public-all.c:149:45: warning: unknown escape sequence: '\g'
../tests/test-is-public-all.c:149:45: warning: unknown escape sequence: '\s'
../tests/test-is-public-all.c:149:45: warning: unknown escape sequence: '\l'
../tests/test-is-public-all.c:149:45: warning: unknown escape sequence: '\p'
../tests/test-is-public-all.c:154:51: error: incomplete universal character name \U
  154 |         if (!(psl4 = psl_load_file(PSL_ASCII_DAFSA))) {
      |                                                   ^
../tests/test-is-public-all.c:154:51: warning: unknown escape sequence: '\g'
../tests/test-is-public-all.c:154:51: warning: unknown escape sequence: '\s'
../tests/test-is-public-all.c:154:51: warning: unknown escape sequence: '\l'
../tests/test-is-public-all.c:154:51: warning: unknown escape sequence: '\p'