allow adding QMAKE FLAGS from env for subdir projects

[amazingfate]

I'm doing packaging for debian these days, and I find there are missing flags when compiling:
https://salsa.debian.org/amazingfate/moonlight-qt/-/jobs/7624590

I'm using qmake6 as buildsystem supported by debhelper. There are very few qt6 projects that is using qmake6, but there is a package doesn't have this issue: https://codeberg.org/ltworf/explosive-c4/src/branch/master/debian/rules.

Now I find that it is sub dir not supporting QMAKE FLAGS passed from parent project.

debhelper is using the following qmake command:

qmake6 -makefile "QMAKE_CFLAGS_RELEASE=-g -O2 -Werror=implicit-function-declaration -ffile-prefix-map=$PWD=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2" "QMAKE_CFLAGS_DEBUG=-g -O2 -Werror=implicit-function-declaration -ffile-prefix-map=$PWD=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2" "QMAKE_CXXFLAGS_RELEASE=-g -O2 -ffile-prefix-map=$PWD=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2" "QMAKE_CXXFLAGS_DEBUG=-g -O2 -ffile-prefix-map=$PWD=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2" "QMAKE_LFLAGS_RELEASE=-Wl,-z,relro -Wl,-z,now" "QMAKE_LFLAGS_DEBUG=-Wl,-z,relro -Wl,-z,now" QMAKE_STRIP=: PREFIX=/usr

But at subdir qmake is not setting flags like QMAKE_CFLAGS_RELEASE with debian's default hardening flags like fstack-protector-strong. So I allow accepting QMAKE FLAGS from env so that all subdir projects can receive the flags set by debhelper.