add notes about libcap

INSTALL.md

@@ -19,6 +19,15 @@ Or if you choose LibreSSL, make sure its `openssl` is first in the $PATH (On Ope
 
 *\* Apple does not include a good enough version of OpenSSL. We officially support the `brew` versions of OpenSSL. If installing OpenSSL from source (untested) make sure to install static libraries.*
 
+### Linux Capabilities
+In order to setuid/setgid on linux, we require that you have libcap headers installed.
+
+```
+sudo apt install libcap-dev # Ubuntu
+sudo dnf install libcap-devel # Fedora
+sudo yum install libcap-devel # CentOS/RHEL
+```
+
 #### Downloading Envelope
 
 https://github.com/workflowproducts/envelope/releases

src/configure

@@ -58,6 +58,13 @@ if test $(uname -s) = "Linux"; then
 	}
 	echo "yes"
 
+	HEADER="sys/capability.h"
+	. scripts/check_header
+	if test "x$HAVE_HEADER" = "x0"; then
+		echo >&2 "Cannot use sys/capability.h! Install libcap-dev (apt) or libcap-devel (yum) and re-run configure."
+		exit 1
+	fi
+
 elif test $(uname -s) = "Darwin"; then
 	LDFLAGS="-lm"
 	echo "Darwin"