SConscript.boot uses $HOME -- but it is not guarenteed to be there nor accessible.

[coldtobi]

Looking into packaging swift for Debian, where builds are generally done e.g. in chroots, scons fails:

make[1]: Entering directory '/build/swift-im-5.0~alpha2+145+g12d031cf8'
scons V=1 qt5=1 optimize=1 debug=1 allow_warnings=1 swiften_dll=1 help2man=1 docbook_xsl=/usr/share/xml/docbook/stylesheet/docbook-xsl docbook_xml=/usr/share/xml/docbook/schema/dtd/4.5 linkflags="-Wl,-z,relro" ccflags="-Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -ffile-prefix-map=/build/swift-im-5.0~alpha2+145+g12d031cf8=. -fstack-protector-strong -Wformat -Werror=format-security"  Swift Swiften debian/swift-im.1 debian/swiften-config.1
scons: Reading SConscript files ...
Checking whether the C++ compiler works... no

Looking into config.log gives a hint:

cat config.log 
file /build/swift-im-5.0~alpha2+145+g12d031cf8/BuildTools/SCons/SConstruct,line 140:
	Configure(confdir = .sconf_temp)
scons: Configure: Checking whether the C++ compiler works... 
.sconf_temp/conftest_737c003a897167642b70f8cdde8a0eaf_0.cpp <-
  |
  |int main(void)
  |{
  |    return 0;
  |}
  |
g++ -o .sconf_temp/conftest_737c003a897167642b70f8cdde8a0eaf_0_b9d11eb9c94b413e6393d4e120436264.o -c -std=c++11 -Wextra -Wall -Wnon-virtual-dtor -Wundef -Wold-style-cast -Wno-long-long -Woverloaded-virtual -Wfloat-equal -Wredundant-decls -Wno-unknown-pragmas -Wdate-time -D_FORTIFY_SOURCE=2 -g -O2 -ffile-prefix-map=/build/swift-im-5.0~alpha2+145+g12d031cf8=. -fstack-protector-strong -Wformat -Werror=format-security -isystem /build/swift-im-5.0~alpha2+145+g12d031cf8/Backport/ -O2 -g -fPIC -DSWIFT_EXPERIMENTAL_FT .sconf_temp/conftest_737c003a897167642b70f8cdde8a0eaf_0.cpp
ccache: error: Failed to create directory /nonexistent/.cache/ccache/tmp: Permission denied
scons: Configure: no

/nonexistent/ is the canonical location for a nonexistant home directories; while building Debian package $HOME is pointing there to catch policy violations during build.

Looking into SConscript.boot I see several locations where "HOME" is referenced…

TL;DR: You build system must not assume that HOME exists and/or is writeable.