Replace bashisms in the configure scripts

[mgorny]

Replace the use of += operator with plain assignment to fix compatibility with shells other than bash. Without this script, the build fails on systems where /bin/sh is not bash (and e.g. dash).

The original code leads to errors such as:

./configure: 14711: PRAGHA_CFLAGS+= ... : not found

since the assignment is wrongly taken to be a command to execute. As a result, flags are not being appended and the build eventually fails with e.g.:

pragha-cmdline.c:33:10: fatal error: libxfce4ui/libxfce4ui.h: No such file or directory

Original bug report: https://bugs.gentoo.org/889618