After building and installing gcc 7.3.0 as cross-compiler, I discovered
that gcc no longer searches ${PREFIX}/include for header files:
```
$ echo "" | /usr/local/djgpp/bin/i586-pc-msdosdjgpp-g++ -E -xc++ -Wp,-v -
# 1 "<stdin>"
# 1 "<built-in>"
# 1 "<command-line>"
# 1 "<stdin>"
#include "..." search starts here:
#include <...> search starts here:
D:/msys64/usr/local/djgpp/lib/gcc/i586-pc-msdosdjgpp/7.3.0/include/c++
D:/msys64/usr/local/djgpp/lib/gcc/i586-pc-msdosdjgpp/7.3.0/include/c++/i586-pc-msdosdjgpp
D:/msys64/usr/local/djgpp/lib/gcc/i586-pc-msdosdjgpp/7.3.0/include/c++/backward
D:/msys64/usr/local/djgpp/lib/gcc/i586-pc-msdosdjgpp/7.3.0/include
D:/msys64/usr/local/djgpp/lib/gcc/i586-pc-msdosdjgpp/7.3.0/include-fixed
D:/msys64/usr/local/djgpp/lib/gcc/i586-pc-msdosdjgpp/7.3.0/../../../../i586-pc-msdosdjgpp/sys-include
D:/msys64/usr/local/djgpp/lib/gcc/i586-pc-msdosdjgpp/7.3.0/../../../../i586-pc-msdosdjgpp/include
End of search list.
$ /usr/local/djgpp/bin/i586-pc-msdosdjgpp-g++ -v
Using built-in specs.
COLLECT_GCC=D:\msys64\usr\local\djgpp\bin\i586-pc-msdosdjgpp-g++.exe
COLLECT_LTO_WRAPPER=D:/msys64/usr/local/djgpp/libexec/gcc/i586-pc-msdosdjgpp/7.3.0/lto-wrapper.exe
Target: i586-pc-msdosdjgpp
Configured with: ../gnu/gcc-7.30/configure --target=i586-pc-msdosdjgpp
--program-prefix=i586-pc-msdosdjgpp- --prefix=/usr/local/djgpp
--disable-nls --enable-libquadmath-support
--enable-version-specific-runtime-libs --enable-languages=c,c++
--enable-fat --disable-plugin --enable-lto --enable-libstdcxx-filesystem-ts
Thread model: single
gcc version 7.3.0 (GCC)
```
Now I haven't used gcc in a while, but I do recall previous versions
used to search the architecture-independent header location too. Is this
a bug/regression, intended behaviour, or am I doing something wrong?
